The two springs suspending the same mass is equivalent to strengthening each of the spring as each of the spring produces a certain restarting force.

Note that the springs are in parallel.

Thus the net spring constant is $k_1+k_2$, whereas if they are in series with only one mass at the end the springs are in series and the net spring constant is $\large\frac{k_1k_2}{(k_1+k_2)}$ and hence is lesser than $k_1+k_2$ as far springs in parallel.

Hence b is the correct answer.