The molecular formula of second homologue in the homologous series of mono carboxylic acids is _________.

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

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 ${\mathrm{CH}}_2$ and molecular mass of $14\mathrm{u}$.

Second homologue : ${\mathrm{CH}}_3\mathrm{COOH}$
$={\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_2$

Hence, the answer is B.