Official Explanation
The credited answer is (D). We know Diamond had high profits before Stibium closed, and we know it was close to bankruptcy after Stibium closed, citing Stibum's closure as one of the primary causes. There, in some way, as a result of Stibium closing, Diamond lost revenue. Consider the opposite of (D): If Stibium closed, and that caused no revenue loss for Diamond, then how on earth could Diamond cite the closure of Stibium as one of the causes of its plummet from high profits to bankruptcy? The opposite of (D) is a scenario that makes no sense, so (D) is an unavoidable inference, very well supported.
Choice (A) is a tempting answer. Would branches of other Diamond stores in other towns have reaped profits, enough to avoid the bankruptcy mentioned? Perhaps. That's certainly a plausible possibility, but we don't know for sure. If we don't know for sure, it's not a good inference. (A) is incorrect.
Choice (B) is way too specific in the kind of assumptions it makes. It seems that Diamond was getting some kind of revenue from Stibium, but was it the management buying perks? or rank-and-file workers buying treats for themselves? We don't know. Anything that spins a highly specific story is too much to infer strictly from the information in the prompt. (B) is incorrect.
Choice (C) makes too many assumptions --- does this specialty store Diamond have direct competitors in the region? if so, are these competitors larger? was "being larger" an advantage in the economic conditions that resulted from Stibium's closure? There are too many things we don't know, so we can draw a clear inference. (C) is incorrect.
Choice (E) is entirely unfounded. We have no idea how big Diamond is, and we have no idea what other employers Apisville might have. (E) is incorrect.
(D) is by far the best answer.