The least common multiple of $4$ and $9$ is ...............

The correct option is D: $36$
Prime factorisation of $4,9$
$\begin{array}{cc}2& 4,9\\ 2& 2,9\\ 3& 1,9\\ 3& 1,3\\ & 1,1\end{array}$
L.C.M. of $4$ and $9=2\times 2\times 3\times 3=36$