Step 1: Understanding the potential formula.
The electric potential \( V \) at a point due to a point charge \( q \) is given by:
\[
V = \frac{kq}{r}
\]
where \( k \) is Coulomb's constant, \( q \) is the charge, and \( r \) is the distance from the charge.
Step 2: Applying the potential for a spherical shell.
For a spherical shell, the potential at a point is determined by the charge at the center and the distance from the center. Given that the potential due to the shell is equivalent to the potential due to a point charge at the center, the result will be \( \frac{qE_n}{2m} \).
Step 3: Conclusion.
Thus, the correct answer is (B) \( \frac{qE_n}{2m} \).