Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : Work done in moving a test charge between two points inside a uniformly charged spherical shell is zero, no matter which path is chosen.
Reason R : Electrostatic potential inside a uniformly charged spherical shell is constant and is same as that on the surface of the shell.
Choose the correct answer :

Electrostatic force is a conservative force, so the work $$W$$ done by (or against) the electric field in taking a test charge $$q_0$$ from point $$A$$ to point $$B$$ is related to the potential difference by
$$W = q_0\,(V_B - V_A)\,\,\,\,\,\,\,\, -(1)$$

For a uniformly charged thin spherical shell of radius $$R$$, the magnitude of the electric field $$E$$ at any point whose distance from the centre is $$r$$ satisfies
$$E = 0 \quad \text{for} \; r \lt R$$
$$E = \dfrac{1}{4\pi\varepsilon_0}\dfrac{Q}{r^{2}} \quad \text{for} \; r \gt R$$

Since $$E = 0$$ everywhere inside the shell, the line integral of $$\mathbf{E}\cdot d\mathbf{l}$$ between any two interior points is zero. Therefore the electrostatic potential throughout the interior is the same constant value, equal to the potential on the surface:
$$V_{\text{inside}} = V_{\text{surface}} = \dfrac{1}{4\pi\varepsilon_0}\dfrac{Q}{R}\,\,\,\,\,\,\,\, -(2)$$

Using $$(2)$$ in $$(1)$$, for any two interior points $$A$$ and $$B$$ we get
$$V_B = V_A \;\; \Longrightarrow \;\; W = q_0\,(V_B - V_A) = 0$$

This result does not depend on the path followed because the electric force is conservative. Hence:

Case 1 : Assertion A
"Work done in moving a test charge between two points inside a uniformly charged spherical shell is zero, no matter which path is chosen."
We have just proven $$W = 0$$, so Assertion A is true.

Case 2 : Reason R
"Electrostatic potential inside a uniformly charged spherical shell is constant and is same as that on the surface of the shell."
Equation $$(2)$$ confirms this, so Reason R is also true.

Because the constancy of potential (Reason R) directly leads to zero potential difference, which in turn makes the work done zero (Assertion A), R is the correct explanation of A.

Therefore the correct choice is Option B: Both A and R are true and R is the correct explanation of A.