Explanation

(c)
When rain is accompanied by thunderstorm, the $N_{2}\ and\ O_{2},$ present in the atmosphere, combine together to give oxides of nitrogen which dissolve is rain water and give nitric acid (a strong acid). Due to presence of acid, the pH of rain water gets slightly lower
$N_{2} + O_{2}\overset{\ Thunderstorm\ }{\rightarrow}\ 2NO$
$2NO + O_{2} \rightarrow 2NO_{2}$
$4NO_{2} + O_{2} + 2H_{2}O \rightarrow 4HNO_{3} + rain\ water \rightarrow acid\ rain$