Hi, let's take a look at the argument. The core is that since problems result at least in part from relationships, to solve problems we must improve those relationships.
This doesn't have to be true. The fact that something is a cause of an effect doesn't mean that we must remove that cause to remove that effect. Perhaps we could improve by simply ending these relationships, or maybe we could improve those relationships incidentally by solely focusing on ourselves.
Ultimately what we need is an answer choice that proves that we need to work on relationships in order to improve. In other words, we need to show that working on external relationships is a necessary condition (or that working on internal problems is not sufficient) to improve.
C tells us that working on external concerns is sufficient in some cases for some improvement. In addition to not being what we needed, all this tells us is that working on external factors helped. We dont know that in these cases the therapist didn't also work on internal factors, and this certainly doesn't help prove a general rule. It also doesn't mean that in these cases working on external solved everything, just that it helped.
D is strangely worded but it does give us what we want. It shows that purely internal work cannot solve all one's problems, that is, it eliminates the flaw that we can fix our relationships by working on ourselves alone. In other words, it shows that some external work is a necessary condition. I actually think this is a slightly weak answer choice for a sufficient assumption question, but it does just do the job we needed it to.
