Step 1: Plan:
Differentiate the relation to get the acceleration.
Step 2: Steps:
$4v^2 = 50 - x^2$. Differentiate with respect to time: $8v\frac{dv}{dt} = -2x\frac{dx}{dt} = -2xv$, so $a = \frac{dv}{dt} = -\frac{x}{4}$.
For SHM, $a = -\omega^2x$, so $\omega^2 = \frac14$ and $T = \frac{2\pi}{1/2} = 4\pi = \frac{88}{7}$ s. Hence $x = 88$.
Final Answer:
The value of $x$ is $88$, option (C).
\[ \boxed{88} \]