A mass M tied to a light string is moving in a vertical circle. The tension in the string at the top is TT and TB at the bottom of the circle. The work done by the tension on M as it moves from bottom to top is:

Tension provides centripetal force in the circular motion. In circular motion work done by force = torque x angle

torque is zero as, centripetal force passes through axis of rotation that is center.

So work done by centripetal force = 0

So work done by tension on M = 0