When can you apply the idea that momentum is constant to solve a problem? (a) When the system is isolated (b) When the system is not isolated but the time interval when the external forces are exerted is very small (c) When the external forces are much smaller than the internal forces

a) when the system is isolated.

Explanation: The law of conservation of momentum states that in an isolated system the vector sum of momentum before collision equals the vector sum after collision.

By isolated system, we mean a system that does not interacts with it environment thus not letting any external force inside.

If the system is isolated, then there will be no external force (e.g pressure) on it, thus the only forces interacting are those of the bodies inside the system which in turns makes it possible to conserve momentum.