a
The two springs are in parallel.
\(f=\frac{1}{2 \pi} \sqrt{\frac{K_{1}+K_{2}}{m}}\)            \(...(i)\)
\(f^{\prime}=\frac{1}{2 \pi} \sqrt{\frac{4 K_{1}+4 K_{2}}{m}}\)
\(=\frac{1}{2 \pi} \sqrt{\frac{4\left(K_{1}+4 K_{2}\right)}{m}}=2\left(\frac{1}{2 \pi} \sqrt{\frac{K_{1}+K_{2}}{m}}\right)\)
\(=2 f \quad\)              from eqn. \((i)\)