Introduction
In order to understand why the electric potential at the center of a cube with constant potential on its surface is also V, we need to consider the concept of electric potential and its relationship with electric fields.

Electric Potential
Electric potential is a scalar quantity that measures the amount of electric potential energy per unit charge at a given point in an electric field. It is denoted by V and is measured in volts (V). The electric potential at a point is defined as the work done per unit charge in bringing a positive test charge from infinity to that point.

Electric Field inside the Cube
Since the cube has a constant electric potential on its surface, this implies that the electric field inside the cube must be zero. This is because if there were an electric field inside the cube, charges would experience a force and move, resulting in a non-zero potential difference and violating the condition of constant potential on the surface.

Electric Potential at the Center of the Cube
The electric potential at the center of the cube can be determined by considering a path from infinity to the center. Since the cube is neutral (no charges inside), the work done in bringing a positive test charge from infinity to the center is zero. This is because there is no electric field inside the cube to do work on the test charge.

Conclusion
Therefore, the electric potential at the center of the cube is the same as the potential on its surface, which is V. This is because there is no electric field inside the cube and no work is done in bringing a positive test charge from infinity to the center. The constant potential on the surface extends to the entire volume of the cube, including the center.