I just had to solve this one and this was difficult.
It's really a hard question to answer, but this is how I explained it to myself:
A negative behavior (need for reassurance) is causing a "dependent stressor" (something you CAN control) to manifest which will increase likelihood for depression.
A dependent stressor is associated with a behavioral cause, something internal that you have control over. "Isolation" is something internal, and something that you are doing unto yourself by driving other people away from you (I think it is pretty important that choice C reads "resulting from others being driven away" implying that it's not others isolating you, but you are the one driving others away and bringing isolation upon yourself). And, importantly, this "isolation" wouldn't have occurred if you were not "reassurance seeking."
For answer choice B, "interpersonal tension caused by the need for reassurance," is not a good answer because there are other people actively involved in this tension as well. You can't have interpersonal tension on your own, there has to be someone else involved in this tension as well.
So, interpersonal tension can be seen partly out of your control (less internal than isolation), which could also be like a "independent stressor" as well.
This is why I think Choice C is the best answer. It is more explicitly a dependent stressor.
