Solution

A Grignard reagent adds to carbon dioxide (dry ice); the carbanion-like carbon attacks the carbon of $CO_2$:
$C_6H_5MgBr + CO_2 \rightarrow C_6H_5COOMgBr$
Acid hydrolysis gives the carboxylic acid:
$C_6H_5COOMgBr + H_3O^+ \rightarrow C_6H_5COOH + Mg(OH)Br$
The acid formed has one carbon more than the Grignard reagent.
So P is benzoic acid, $C_6H_5COOH$.

Concept Carboxylation of Grignard reagent

Aldehydes, Ketones and Carboxylic Acids in past papers

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