Explanation

(a)
Nitration or aromatic compounds takes place by an electrophile. The electrophile will be more attracted towards electron rich position in benzene ring. Hence, electron donating groups will be easily nitrated.
Toluene will be most easily nitrated among these compounds due to presence of electron donating group$\ (i.e.,\ CH_{3}).$
Nitrobenzene will be most slowly nitrated due to the presence of electron withdrawing group $(i.e.,\ NO_{2})$.
$CH_{3}NO_{2}$ will be formed by free radical substitution of $CH_{4}$