Step 1: Understanding the Concept:
For a charged spherical shell, the electric field inside is zero. This means the electric potential remains constant at all points inside and is equal to the potential on the surface.
Step 2: Detailed Explanation:
The potential on the surface ($r=R$) is $V = \frac{Q}{4\pi\varepsilon_0 R}$.
Since the potential is constant inside the shell ($0 \leq r \leq R$), the potential at $r = R/2$ is exactly the same as the potential on the surface.
$$V_{inside} = \frac{Q}{4\pi\varepsilon_0 R}$$
Step 3: Final Answer:
The correct option is (a).