Question

The chief reason that family and friends are frequent targets of aggression is that

A. frustration-producing obstacles are often unclear.  B. they're convenient.

 C. as obstacles, they're unremovable.   D. they cause the most frustration

Answer 1

I think the most likely reason that family and friends frequently become targets of aggression is that "they're there", that is, they are present when you come home from work, or when something happens and might just accidentally encounter us in a bad mood.

This can be phrased as "they are convenient"- they are there to be the object of frustration.