Since the element B belongs to atomic number 17, it must be chlorine.
The element which burns golden flame in air is sodium. Therefore, the element A must be sodium.
When element sodium and chlorine reacts, it forms sodium chloride, the compound C. The compound C on electrolysis gives sodium hydroxide and hydrogen gas.
Thus, A = Na; B = CI₂; C = NaCI; D = NaOH.
         Chemical Reactions involved:
              \(2Na+{ CI }_{ 2 }\rightarrow 2NaCI\)
\(2NaCI(aq)+{ 2H }_{ 2 }O(I)\rightarrow 2NaOH(aq)+{ CI }_{ 2 }(g)+{ H }_{ 2 }(g)\)

Since the metal X react with cold water, it must be high in the reactivity series i.e., sodium or potassium.
X is alkali metal, Na or K.
Since the metal Y reacts with hot water, it must be middle in the reactivity series i.e., magnesium or calcium.
Y is alkaline earth metal, Mg or Ca.
Since the metal X reacts with steam only, it must be low in the reactivity series i.e., iron Z is Fe.
Increasing reactivity series : Na > Mg > Fe.

(a) Due to the formation of a layer of oxide i.e., Al₂O₃
(b) Na or Mg is more reactive metals as compared to carbon.
(c) In solid NaCI, the movement of ions is not possible due to its rigid structure but in aqueous solution or molten state, the ions can move freely.
(d) To protect from corrosion.
(e) They are highly reactive.

Since the non-metal A is the largest costituent of air, it must be Nitrogen. When nitrogen is heated with hydrogen in the ratio of 1 : 3, it gives ammonia in presence of iron as a catalyst [Haber's process]. When nitrogen is heated with oxygen, it gives nitric oxide. When this oxide is passed through water in presence of air, it gives nitric acid which acts as a strong oxidising agent.
(a) A - N₂; B - NH₃; C - NO; D - HNO₃
(b) Element A belongs to Group - 15 of the Periodic Table.