The decreasing order of reactivity of

(I) m-nitrobenzene;

(II) 2, 4, 6-trinitro bromobenzene

(III) p-nitro bromobenzene

(IV) 2, 4-dinitro bromobenzene

II > IV > III > I

The correct answer is option 2. II > IV > III > I.

The reactivity of a compound towards electrophilic substitution reactions depends on the number and position of the electron-withdrawing groups (EWGs) on the benzene ring. The more EWGs a compound has, the more reactive it is. The position of the EWGs also affects reactivity, with ortho and para substituents being more reactive than meta substituents.

In the case of the compounds listed, the order of reactivity is as follows:

\[II > IV > III > I\]

2, 4, 6-trinitrobromobenzene (II) has three EWGs (NO₂) in the ortho and para positions, making it the most reactive compound.

2, 4-dinitrobromobenzene (IV) has two EWGs (NO₂) in the ortho and para positions, making it the second most reactive compound.

p-nitrobromobenzene (III) has one EWG (NO₂) in the para position, making it the third most reactive compound.

m-nitrobromobenzene (I) has one EWG (NO₂) in the meta position, making it the least reactive compound.

Therefore, the decreasing order of reactivity is II > IV > III > I.