When is the angular momentum of a system constant? A) Only when its total kinetic energy is constant. B) Only when no net external force acts on the system. C) Only when the linear momentum and the energy are constant. D) Only when no net external torque acts on the system. E) Only when the moment of inertia is constant.

The correct answer is letter "D": Only when no net external torque acts on the system.

Explanation:

An object's total angular momentum is the sum of the spin and angular momentum of the orbit. Within a system where there is no net external torque or the torque equals zero - also known as a closed system, the total angular momentum is said to be constant.