This is an oxymercuration-demercuration reaction. Anisole (methoxybenzene) reacts with mercuric acetate in the presence of water, followed by reduction. The mercury adds to the aromatic ring at the para position (due to the activating methoxy group directing ortho/para), and subsequent demercuration gives the para-bromomethoxybenzene product shown in option D along with H₂ as a byproduct.
JEE Advanced · 2010 · 11 Apr 2010 · Paper 1 · Single correct option