The end product C is ethyl alcohol.
CH3BrKCN→CH3−CNAH3O+→CH3−COOHBLiAlH4→CH3−CH2−OHC
Methyl bromide reacts with KCN to form acetonitrile. Br atom is replaced with CN group. Hydrolysis of cyano group gives carboxylic group. Reduction of carboxylic group gives hydroxyl group.