Topic: Oscillatory Motion
Concepts: Forces in mechanics, Mass-spring system, Simple harmonic motion

Solution:

Suppose the springs have equilibrium length $l$ and spring constant $k$. If we displace the mass $m$ by distance $x$ parallel to the spring, the spring force $F$ is given by

Then the restoring force is

For finite $x$, the magnitude of this restoring force is less than the simple harmonic force $F_{\text{SHM}}=-2kx$ so the actual acceleration of the mass is less than the assumed simple harmonic acceleration. Hence, the period $T$ is longer than the SHM period

As $x \rightarrow \infty, F_r \rightarrow F_{\text{SHM}}$ so $T \rightarrow T_{\text{SHM}}$. Since $T>T_{\text{SHM}}$ at small displacements, $T$ decreases as $x$ increases. Thus, the answer is C.