What is angular momentum, and how is it conserved?

Angular momentum is the product of moment of inertia and angular velocity, and it is conserved in a closed system with no external torques.

Angular momentum is the sum of linear momenta in a system, and it is conserved when external forces are zero.

Angular momentum is the rate of change of linear momentum, and it is conserved only when friction is absent.

Angular momentum is the product of mass and velocity, and it is conserved in a system with constant velocity.