A uniform cylinder of length $L$ and mass $M$ having cross-sectional area $A$ is suspended with its vertical length, from a fixed point by a massless spring, such that it is half submerged in a liquid of density $d$ at equilibrium position. When released, it starts oscillating vertically with a small amplitude. If the force constant of the spring is $k$, the frequency of oscillation of the cylinder is
$$
\frac{1}{2 \pi}\left(\frac{k-A d g}{M}\right)^{1 / 2} \quad \text { (b) } \frac{1}{2 \pi}\left(\frac{k+A d g}{M}\right)^{1 / 2}
$$
(c) $\frac{1}{2 \pi}\left(\frac{k-d g L}{M}\right)^{1 / 2}$
(d) $\frac{1}{2 \pi}\left(\frac{k+A g L}{A d g}\right)^{1 / 2}$