The principle of conservation of angular momentum for a rigid body is applicable when:

The principle of conservation of angular momentum for a rigid body is applicable when: Correct Answer Resultant external moment is zero

Concept:

Angular momentum is a vector quantity.

The magnitude of the angular momentum of an orbiting object is equal to its linear momentum (product of its mass ‘m' and linear velocity ‘v’) times the perpendicular distance ‘r’  from the center of rotation to a line drawn in the direction of its instantaneous motion and passing through the object’s center of gravity.

Angular momentum = m × v × r

Conservation of angular momentum:A rigid spinning object continues to spin at a constant rate and with a fixed orientation unless influenced by the application of external torque.

From the above expression,

It is clear that if resultant external moment (m v) is zero, angular momentum is conserved.