I was confused about this at first too, but looking closer at the specific phrasing in the stimulus and Answer Choice B helps. The conclusion is basically saying that both doctrines are mistaken, bc there must be events that were caused by both economic & psychological factors. It's assuming that doctrines 1 & 2 mutually exclude the possibility of the other.

1st doctrine says that an explanation has to "appeal" to economics, but not necessarily that economics is the ONLY thing that an explanation can appeal to. So, an acceptable 1st doctrine could have a psychological explanation that also appeals to economics. That's why assuming A would then make the argument follow logically: it would ensure that doctrine 1 precludes doctrine 2.

2nd doctrine says that we can use psychology to explain historical events, ESPECIALLY psychology influenced by childhood. Again, it doesn't necessarily say that psychology is the ONLY possible historical explanation. Paying close attention to the wording of B, all it's asking us to assume is that childhood experience is the ONLY thing affecting psychological explanations in doctrine 2. That narrows the scope of what we can say influences psychology, but doesn't affect whether doctrine 2 excludes economic factors.

If it's easier to do so, think of it in terms of negating A and B. If you negate B, and the 2nd doctrine also places importance on other experiences (ex: adolescent, adult, etc) that does not make the conclusion impossible. If you negate A, and the 1st doctrine CAN include non-economic factors (i.e. it could accept a joint psychological & economic explanation) then the conclusion is impossible: the doctrines are not mistaken, because they would in fact not be mutually exclusive.