A system shown in figure. consists of a massless pulley, a spring of force constant $k$ and a block of mass $m .$ If block is just slightly displaced vertically down from its equilibrium position and released then the period of vertical oscillations is :
(a) $T: \pi \sqrt{\left(\frac{m}{4 k}\right)}$
(b) $T=2 \pi \sqrt{\left(\frac{m}{4 k}\right)}$
(c) $T=2 \pi \sqrt{\left(\frac{m}{2 k}\right)}$
(d) $T=2 \pi \sqrt{\left(\frac{m}{3 k}\right)}$