(d) KE=PE $\Rightarrow$ $\frac{1}{2}\ m\omega^{2}\left( a^{2} - y^{2} \right) = \frac{1}{2}\ m\omega^{2}y^{2}$ $\Rightarrow$ $y = \frac{a}{\sqrt{2}}$
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.