The electron in a hydrogen atom makes a transition n₁ → n₂, where n₁ and n₂ are the principal quantum numbers of the two states. Assume the Bohr model to be valid. The time period of the electron in the initial state is eight times that in the final state. The possible values of n₁ and n₂ are

$n_1=4, n_2=2$

$T \propto n^3$. Given $T_{n_1}=8 T_{n_2}$, hence $n_1=2 n_2$

Therefore, option (a) and (d) both are correct.