$ \Rightarrow PV = (P + \Delta P)(V - \Delta V)$ 

$ \Rightarrow PV = PV + \Delta PV - P\Delta V - \Delta P\Delta V$ 

$ \Rightarrow \Delta P.V - P.\Delta V = 0$ (neglecting $\Delta P.\Delta V)$

$\Delta P(Ah) = P(Ax)$

$ \Rightarrow \Delta P = \frac{{P.x}}{h}$

This excess pressure is responsible for providing the restoring force $(F)$ to the piston of mass $M$. 

Hence $F = \Delta P.A = \frac{{PAx}}{h}$ 

Comparing it with $|F| = kx$

$\Rightarrow k = M{\omega ^2} = \frac{{PA}}{h}$ 

$ \Rightarrow \omega = \sqrt {\frac{{PA}}{{Mh}}} $

$\Rightarrow T = 2\pi \sqrt {\frac{{Mh}}{{PA}}} $ 

Short trick : by checking the options dimensionally. Option (a) is correct.