Propanoic acid is converted to its acid chloride with $SOCl X_{2}$ , which is treated with excess diazomethane. The product is then heated with $Ag X_{2} O$ and water. The final acid is

Answer: D

CH X_{3} CH X_{2} COCl

and

CH X_{2} N X_{2}

give the diazo ketone

CH X_{3} CH X_{2} COCHN X_{2}

. With

Ag X_{2} O

it loses

N X_{2}

to form a keto-carbene, which undergoes Wolff rearrangement to the ketene

CH X_{3} CH X_{2} CH = C = O

. Water adds to give

CH X_{3} CH X_{2} CH X_{2} COOH

, so the chain is lengthened by one carbon.