The molecular formula of second homologue in the homologous series...

The general molecular formula for carboxylic acid is ${C}_{n}{H}_{2n + 1}COOH$.

In organic chemistry, a homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the members of the series can be branched or unbranched, or differ by molecular formula of ${CH}_{2}$ and molecular mass of $14u$.

Second homologue : ${CH}_{3}COOH$
$= {C}_{2}{H}_{4}{O}_{2}$

Hence, the answer is B.