When can you apply the idea that momentum is constant to solve a problem?

It seems that you have missed the necessary options for us to answer this question, so I had to look for it. Anyway, here is the answer. You can apply the idea that momentum is constant to solve a problem when the system is not isolated but the time interval when the external forces are exerted is very small. Hope this helps.

Ithink the answer is a

B) as the temperature increases, the kinetic energy of the particles increases.|-/