$l$ 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
$a b c \times d \times g=b c l \times 1 \times g$
$\Rightarrow l=d a$
$\Rightarrow T=2 \pi \sqrt{\frac{d a}{g}}$

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