here, $x=\frac{A}{2}=A \sin \left(\frac{2 \pi}{T}\right) t$
or, $\sin \left(\frac{2 \pi}{T}\right) t=\frac{1}{2}=\sin \frac{\pi}{6} \Rightarrow t=\frac{T}{12}$
$\frac{d x}{d t}=\frac{2 \pi}{T} A \cos \left(\frac{2 \pi}{T}\right) t$
$\frac{d^{2} x}{d t^{2}}=-\left(\frac{2 \pi}{T}\right)^{2} A \sin \left(\frac{2 \pi}{T}\right) t$
so, $v_{\max }=v_{0}=\left(\frac{2 \pi}{T}\right) A, a_{\max }=a_{0}=\left(\frac{2 \pi}{T}\right)^{2} A$
now at $t=T / 12,$ the velocity, $v=\frac{d x}{d t}=\frac{2 \pi}{T} A \cos \left(\frac{2 \pi}{T}\right) \frac{T}{12}$
$=\frac{2 \pi}{T} A \cos \frac{\pi}{6}=\frac{2 \pi}{T} A \frac{\sqrt{3}}{2}$
$\therefore v=\sqrt{3} v_{0}$
at $t=T / 12,$ the acceleration, $a=\frac{d^{2} x}{d t^{2}}=\left(\frac{2 \pi}{T}\right)^{2} A \sin \left(\frac{2 \pi}{T}\right) \frac{T}{12}$
$=\frac{2 \pi}{T} A \sin \frac{\pi}{6}=\frac{2 \pi}{T} A \frac{1}{2}$
$\therefore a=\frac{a_{0}}{2}$
$(A)$ the speed of the particle when it returns to its equilibrium position is $u_0$.
$(B)$ the time at which the particle passes through the equilibrium position for the first time is $t=\pi \sqrt{\frac{ m }{ k }}$.
$(C)$ the time at which the maximum compression of the spring occurs is $t =\frac{4 \pi}{3} \sqrt{\frac{ m }{ k }}$.
$(D)$ the time at which the particle passes througout the equilibrium position for the second time is $t=\frac{5 \pi}{3} \sqrt{\frac{ m }{ k }}$.