Which of the following expressions corresponds to simple harmonic motion along a straight line, where x is the displacement and a, b, c are positive constants?

In linear S.H.M., the restoring force acting on particle should always be proportional to the displacement of the particle and directed towards the equilibrium position.

$\begin{array}{l}\text{ i.e., F}\propto x\text{ }\\ \therefore \text{ F}=-bx\text{ where b is a positive constant. }\end{array}$