$\text{Let the required number of days be x.}$
$\text{Less men, More days (Indirect proportion)}$
$\therefore$ $\text{ $42$ : $56$ :: $24$ : x}$
$\Leftrightarrow$ $42*x$ $=$ $56*24$
$\Leftrightarrow$ $x$ $=$ $(\frac{56*24}{42})$
$\Leftrightarrow$ $x$ $=$ $32$
$\textrm{Option B.}$