Why is A correct?

Answer D is incorrect because it states that the argument proceeds by inferring the existence of “a common cause” of two phenomena. In the passage, there are two phenomena discussed: crop success and birth weights. There is no “common cause” of these two phenomena discussed in the passage (the mention of “amount of food” in the final sentence is just a different way of referring to the crop success phenomenon.) Answer A is correct because, as you correctly note in your question, the author is stating a hypothesis at the end. But this hypothesis is about a correlation between the two phenomena, without any “common cause.” I hope this helps. Please let us know if you have any more questions.

Why is this a and not b? And how can I improve on these kinds of questions

@Aidyn-Carlson, Happy to help. Let's take a look at (A) and (B). (B) says, "inferring from the claim that two phenomena have fluctuated together that one of those phenomena must be the sole cause of the other." The argument's conclusion doesn't make the claim that the success of crops and mothers' access to food is the sole cause of something; rather, the conclusion instead states that one depends largely on the other. For this reason, we can get rid of (B). (A) says, "inferring from a claimed correlation between two phenomena that two other phenomena are causally connected to one another." The phenomena that are correlated in the stimulus are birthweights and crops' success. The phenomena that the argument claims are causally connected to one another are the health of the babies and the mothers' availability of food to the mother. In looking at (A), it does a great job in stating what's going on in the argument in general terms. The argument is taking a correlation between two things and inferring causation between two other things, so (A) is the correct answer choice. Does this make sense? Let us know if you have any more questions!

@Aidyn-Carlson, to improve on these questions, you've got to practice a ton. The more you do these questions and diligently review, the better you'll be able to understand how to tackle these well.