All of these

Consumer preference

Freedom of enterprise

Interrelated to each other

Income from the railways (as it refers to an individual industry).

MRT is the rate at which the units of one good have to be sacrificed to produce one more unit of the other good in a two goods economy. Suppose an economy produces only two goods X and Y. Further suppose that by employing these resources fully and officiently, the economy produces 1X + 10Y. If the economy decides to produce 2X, it has to cut down production of Y by 2 units. Then 2Y is the opportunity cost of producing 1X. Then 2Y:1X is the MRT.

There are two properties of a production possibilities curve.
1. Downward sloping : It is because as more quantity of one good is produced some quantity of the other good must be sacrificed.
2. Concave to the origin : It is because the marginal rate of transformation increases as more of one good is produced.

The production possibility curve being concave means that MRT increases as we move downward along the curve. MRT increases because it is assumed that no resource is equally efficient in production of all goods. As resources are transferred from one good to another, less & less efficient resources have to be employed. This raises cost and raises MRT.

An economy has millions of commodities to produce.  It has to decide which goods and services to be produced and that should be in sufficient quantity, as all the goods cannot be produced due to scarcity of resources. e.g. an economy has to decide whether to produce luxury goods or inferior goods, capital goods or consumer goods.
This problem has two dimensions:
(i) Types of goods to be produced.
(ii) Quantity of goods to be produced.

The problem of 'how to produce' is a problem relating to choice of techniques.  Broadly, it is the problem of deciding the techniques of production which are to be used.
There are two type of techniques of production, which are as follow:
(i) Labour intensive technique It is that technique in which labour is used more than capital.
(ii) Capital intensive technique It is that technique in which capital is used more than labour.

This is very correct to say that only scarce goods are high in price.  This can be supported by the water, diamond paradox.  Though water is very useful for all living being, however, it is very low priced as it is available in plenty.  On the other hand, diamond which is not so useful is still very high in price due to its relative scarcity.

Economics is concerned with the study of economic problems at the level of an economy as a whole, on one side and on the other, it is concerned with the study of an individual too. The subject matter of economics is studied under two broad branches viz microeconomics and macroeconomics.
Vital theories or studies of microeconomics are:
(i) Theory of consumer behaviour.
(ii) Theory of price.
(iii) Theory of producer behaviour.
Vital theories or studies of macroeconomics are:
(i)Theory related to equilibrium level of output and employment.
(ii) Theory related to inflationary and deflationary gap in the economy.
(iii) Theory of multiplier.
(iv) Study of government budget.
(v) Study of exchange rate and Balance of Payments (BoP).

Examples of microvariables are:-
(i) Consumer's demand
(ii) Market price of a commodity
(iii) Firm's output (supply)
Examples of macrovariables are:
(i) Aggregate Demand in the economy.
(ii) General price level in the economy.
(iii) Aggregate Supply in the economy.

The properties of a production possibility curve are:
(a) Downward sloping from left to right: Resources are given and fixed, so increase in production of one commodity is possible only if production of another commodity is reduced.
(b) Concave from the origin: Production possibility curve is concave from the
origin due to increasing marginal opportunity cost.

Production possibility curve is based on the following assumptions:
(a) The economy is producing only two goods.
(b) The resources available with the economy are given and fixed:
(c) Resources are not equally suitable for production of both the goods but can be shifted from production of one good to the other.
(d) All the available resources are fully and efficiently employed and there is no wastage
(e) Technology available with the economy is also given and fixed.

A production possibility curve depicts different combinations of two commodities that an economy can produce with fuller and efficient utilisation of given scarce resources. On the basis of the given diagram, as we move downwards from point A to B, B to C, and so on, (assuming full and efficient employment of resources), the PPC must be concave, because the marginal rate of transformation (MRT) increases continuously as more and more of one good is produced at the cost of reducing the amount of the other good

Increasing sacrifice of Green Chilli for each unit of sugar produced gives concave shape to the PP curve as shown with the help of rate of sacrifice or MOC..
                 

An economic problem mainly arises on account of the scarcity of the resources available to satisfy the unlimited human wants. To a large extent, if the resources were not limited, the economic problem would not have arisen, because every want could be satisfied, i.e., everyone could have everything. But this indeed is not true as the reality is that relative scarcity will continue to prevail and the other related problem -of 'choice' would always exist. One is always confronted with the problem of what to choose and what to give up, i.e., sacrifice. Thus, scarcity, being a relative term, will always exist and so would an economic problem.