$\text{Let the required number of days be x.}$

$\text{Less men, More days (Indirect proportion)}$

$\therefore$ $\text{ $42$ : $56$ :: $24$ : x}$

$\Leftrightarrow$ $42*x$ $=$ $56*24$

$\Leftrightarrow$ $x$ $=$ $(\frac{56*24}{42})$

$\Leftrightarrow$ $x$ $=$ $32$

$\textrm{Option B.}$