At what point in the swing of the pendulum is the potential energy completely converted into kinetic energy

The correct answer is B

Explanation:

The motion of a pendulum is a classic example of mechanical energy conservation. So you can assume, ignoring  air resistance, that total mechanical energy is:

Where K is kinetic energy, U is potential energy and E is the resulting mechanical energy.

You know that:

From this, you can deduce that for potential energy to be transformed entirely to kinetic energy the height of the pendulum mass must be zero and traveling at its maximum velocity, wich is depicted as B in the attached picture.