c
On displacing \(m\) to right by \(x\)

\(F =-\left( k _1 x+ k _2 x \right)=-\left( k _1+ k _2\right) x\)

\(a =\frac{ F }{ m }=-\left(\frac{ k _1+ k _2}{ m }\right) x =-\omega^2 x\)

\(\therefore \omega=\sqrt{\frac{ k _1+ k _2}{ m }} \Rightarrow T=\frac{2 \pi}{\omega}=2 \pi \sqrt{\frac{ m }{ k _1+ k _2}}\)