Nitration is an electrophilic substitution reaction. So, the group which increase the electron density on benzene ring will increase the electrophilic substitution reaction.

Here $$-$$ OCH₃ group shows + R effect, but $$-$$ CH₃ group shows + I effect. As + R effect is more effective than + I effect so $$-$$ OCH₃ will increase more electron density on benzene ring.

$$-$$ Cl has strong electron withdrawing effect ($$-$$ I). So it decrease electron density on the benzene ring.

In acidic medium, aniline form anilinium ion.


This anilinium ion is least reactive as in benzene ring electron density is least.

$$ \therefore $$   The increasing order is (a) < (b) < (d) < (c)