Option ( A) is correct due to following reasons
1. (D) does not support the given molecular formula so rejected.
2. All of (A) ,(B) and (C) can produce AgCl when treated witthAgNO3.as they all contain - CH2Cl group as side chain stated to the benzene ring. These three also support the formation of a compound of molecular formula C7H7OCl.
3. But (A) only supports the formation of one mono nitro isomer as in (A) there exist 4 equivalent H atoms in the ring and replacement of any one of them by nitro group forms the same mono nitro derivative.