Step 1: Understanding the formula.
The frequency \( f \) of simple harmonic motion is related to the angular frequency \( \omega \) by:
\[
f = \frac{\omega}{2\pi}
\]
Given that the frequency is \( \frac{7}{2\pi} \), we can find the angular frequency \( \omega \) as:
\[
\omega = 7 \, \text{rad/s}
\]
Step 2: Relating angular frequency to the equation of motion.
The equation of motion for S.H.M. is:
\[
\frac{d^2x}{dt^2} + \omega^2 x = 0
\]
Substituting \( \omega = 7 \), we get:
\[
\frac{d^2x}{dt^2} + 49x = 0
\]
Step 3: Conclusion.
Thus, the differential equation for the given frequency is \( \frac{d^2x}{dt^2} + 49x = 0 \), corresponding to option (C).