D is irrelevant and not backed by anything in the passage. To understand why B is correct, ask yourself what the key tenets of the SG hypothesis are. The SG hypothesis basically says that a depressive individual will exhibit behavior that leads to more stressful situations occurring to him/herself than non-depressive individuals. These stressful situations then cause more depression, which causes them to exhibit that behavior even more. It's like a positive feedback loop. So to break that loop, the idea is that you prevent depressive episodes. If, when a person gets depressed, they upregulate behavior that leads to stressful situations, then by preventing depressive episodes from happening to them, you prevent that upregulation.