Answer:"B. "hang out"
Explanation:hang out is a casual term used mostly by young people or used when someone is talking to a friend ,or their peers or people that they are close with which makes it an informal phrase because it is one term one can not use in formal setting like when they are talking to their boss, one wouldn't use this phrase. It actual means going out, relaxing and enjoying yourself with friends . In this context we can see that the person is writing to his friend , with whom they are doing a group work.