To solve this problem, we need to match each process or substance with its corresponding product or reactant. Let's analyze each pair:$\\$• (a) Pure nitrogen is produced from sodium azide or barium azide. When sodium azide $($NaN$_3)$ or barium azide $($Ba(N$_3)_2)$ decomposes, it releases nitrogen gas:$\\$2NaN$_3 \rightarrow 2Na + 3$N$_2 \\$Therefore, (a) matches with (iv).$\\ \\$• (b) The Haber process is used to synthesize ammonia from nitrogen and hydrogen gases:$\\$N$_2 + 3$H$_2 \rightarrow 2$NH$_3 \\$Therefore, (b) matches with (iii).$\\ \\$• (c) The Contact process is used for the industrial production of sulphuric acid:$\\$2SO$_2 + O_2 \xrightarrow{{V}_2{O}_5} 2SO_3 \\$SO$_3 + H_2O \rightarrow H_2SO_4 \\$Therefore, (c) matches with (ii).$\\ \\$• (d) Deacon’s process is used to produce chlorine from hydrogen chloride and oxygen:$\\$4HCl + O$_2 \rightarrow 2Cl_2 + 2H_2O \\$Therefore, (d) matches with (i).$\\ \\$Now, let's match these findings with the given options:$\\$Option 1: (a)→(i), (b)→(ii), (c)→(iii), (d)→(iv)$\\$Option 2: (a)→(ii), (b)→(iv), (c)→(i), (d)→(iii)$\\$Option 3: (a)→(iii), (b)→(iv), (c)→(ii), (d)→(i)$\\$Option 4: (a)→(iv), (b)→(iii), (c)→(ii), (d)→(i)$\\ \\$The correct matching is:$\\$(a)→(iv), (b)→(iii), (c)→(ii), (d)→(i)$\\$Therefore, the correct option is 4.