Explanation

(a)
Platinum acts as catalyst in the oxidation of ammonia to form nitric oxide .This reaction is used in the ostwald ‘s method of nitric acid preparation.
4NH_{3}+5O_{2}$\longrightarrow$4NO+6H_{2}O
2NO+O_{2}⟶2NO_{2}
4NO_{2}+O_{2}+2H_{2}O⟶4HNO_{3}