(a) Speed is defined as the distance travelled by a body per unit time.
(b) The SI unit of speed is m s^-1
(c) Average speed is the total distance travelled by a body, divided by the total time taken to cover that distance.
(d) No, the average speed of an object can't be negative.

A body is said to move along a straight line if its position keeps on changing with time.
Examples of motion in a straight line or rectilinear motion:
(i) A bus moving on a straight road;
(ii) A train moving on a straight track;
(iii) A runner running along a straight track.
(iv) A ball moving along a straight path ; and
(v) An object falling vertically downwards towards the surface of the earth.

Different types of Motions:
1.Translatory Motion When a body moves as a whole along a straight or curved path, it is said to be in translatory motion.It is again of two types:
(i) Rectilinear motion Here a body moves as a whole along a straight path.For example a train moving on straight rails has translatory rectilinear motion.
(ii) Curvilinear motion.Here a body moves as a whole along a curved path For example motion of a bicycle taking a turn along a curved path.
2.Rotatory motion.When a body rotates about a fixed point or axis , it has rotatory motion For example motion of a flywheel about a shaft.
3.Vibratory or oscillatory motion. When a body moves to and from about a mean position again and again it has vibratory or oscillatory motion For example the motion of the pendulum of a wall clock. 
4.Complex motion. Sometimes ,the motion of a body may be a combination of more than one types of motion For example a ball rolling down an inclined plane has botth translatoruy and rotatory motions.

(a) Rest and motion are relative terms/
(b) Yes. An object may be at rest relative to one object and at the same time it may be in motion relative to another object.
(c) Moving.

When an object is at rest, its distance-time graph is a straight line parallel to the time axis.

A straight line parallel to the x-axis in a distance-time graph indicates that with a change in time, there is no change in the position of the object. Thus, the object is at rest.

(i) The nature of the distance-time graphs for the uniform motion of an object is a straight line. (Follow the below graph 1.)
(ii) The nature of the time distance-time graph for non-uniform motion is a curved line. Distance-time graph for a car moving with non-uniform speed. (Follow the below graph 2.)

Initial speed,u = 0
Final speed,v = \(40\frac { km }{ h } =\frac { 40X1000m }{ 3600s } =\frac { 100 }{ 9 } ms^{ -1 }\)
Time,t = 10 min = 600 s
Acceleration,a = \(\frac { v-u }{ t } =\frac { \frac { 100 }{ 9 } -0 }{ 600 } =\frac { 100 }{ 9X600 } ms^{ -2 }\)
\(=\frac { 1 }{ 54 } ms^{ -2 }\) 

Retardation. When the velocity of a body decreases, its acceleration is negative Negative acceleration is called 'retardation' or 'deceleration' When a stone is thrown upwards, it is under retardation. Similarly, when a bus approaches a bus stop, its motion gets retarded.

Speed of a body needs only magnitude and not direction for its complete description while the velocity needs both magnitude and direction for its complete description so speed is a scalar quantity while velocity is a vector is a vector quantity for example when a car moves wit a speed of 60km/h in the direction 45 degree south of west,we can say that the car moves with a velocity of 60 km/h 45 degree south of west Here the word velocity replaces both the words speed and direction

Distance covered after 2 complete rounds
= 2 x Circumference
= 2x 2\(\pi \) (2R) = 8 \(\pi \)R
After 2 complete rounds, the body comes back to its initial position
\(\therefore \) Displacement = 0.

Non-uniform motion .If an object covers unequal distances in equal intervals of time,it is said to be in non-uniform motion. Most of the motions seen in our daily life are non-uniform. For example,if we drop a ball  from the roof of a building we will note that the ball covers 4.9 m in the 1st second 14.7 m in the 2nd second 24.5 m in the 3rd second , and so on That is the ball covers increasingly larger distances in successive seconds as it falls down. Thus, the motion of a freely falling body is non-uniform
Other examples of non-uniform motion:
(i) A stone dropped from the top of a building
(ii) A ball thrown vertically upwards
(iii) The motion of a train as it leaves the station
(iv) The motion of a bus as it approaches a bus-stop
(v) The motion of a ball rolling down an inclined plane.

\(v_{ av }=\frac { s_{ 1 }+s_{ 2 } }{ t_{ 1 }+t_{ 2 } } =\frac { s }{ \frac { s_{ 1 } }{ v_{ 2 } } +\frac { s_{ 2 } }{ v_{ 2 } } } \)
\(40=\frac { 100 }{ \frac { 30 }{ 30 } +\frac { 70 }{ v_{ 2 } } } \)
\(=1+\frac { 70 }{ v_{ 2 } } =\frac { 100 }{ 40 } =\frac { 5 }{ 2 } \)
\(\frac { 7 }{ v_{ 2 } } =\frac { 5 }{ 2 } -1=\frac { 3 }{ 2 } \)
\(v_{ 2 }=\frac { 7X2 }{ 3 } =\frac { 14 }{ 3 } =4.67\quad km\quad h^{ -1 }\)