d
A mass \(M\) is suspended from a massless spring of spring constant \(k\) as shown in figure \((a)\). Then,

Time period of oscillation is

\(T=2 \pi \sqrt{\frac{M}{k}}\)

When a another mass \(M\) is also suspended with it as shown in figure \((b).\) 

Then,

Time period of oscillation is

\(T^{\prime} =2 \pi \sqrt{\frac{M+M}{k}}=2 \pi \sqrt{\frac{2 M}{k}}\)

\(=\sqrt{2}(2 \pi \sqrt{\frac{M}{k}})=\sqrt{2} T \quad(\text { Using }(\mathrm{i}))\)