Answer
$\text{At }x=0 \text{ the acceleration of the shadow is 0}$
Work Step by Step
$\text{The shadow performs Simple Harmonic Motion:}$
$$\therefore F = -kx$$
$$\therefore a = -\frac{kx}{m}$$
$\text{Therefore, at }x=0 \text{ the acceleration of the shadow is 0}$
