Two springs of force constants k1 and k2 are connected as shown in the figure. The time period of vertical oscillations of mass m is given by
2π√m(k1+k2)
The two springs are connected in parallel. Therefore, the force constant of the combination, k′=k1+k2
Now, time period, T=2π√mk′=2π√m(k1+k2)
Hence, the correct choice is (a).