d
Time period of $SHM$ of small vertical oscillations in a liquid is given by $T=2 \pi \sqrt{\frac{l}{g}, \text { where }}$
$l$ is the length of $cube/cylinder$ dipped in the water.
So according to law of floatation,
weight of the cube $=$ weight of the water displaced
$a b c \times d \times g=b c l \times 1 \times g$
$\Rightarrow l=d a$
$\Rightarrow T=2 \pi \sqrt{\frac{d a}{g}}$