Thanks for the question! This question is asking us for an answer choice that’s most strongly supported by the information presented to us in the passage, which is just a fact set about specially bred aquarium fish. Let’s take a look at (D) first, which tells us that ordinary fish tend not to have elaborate tails or strangely shaped fins. Take a look at the second sentence of the stimulus. We’re told here that specially bred fish can’t reach food as quickly as ordinary fish that compete with them for food, since the specially bred fish are hampered by elaborate tails or strangely shaped fins. Well, if ordinary fish had elaborate tails or strangely shaped fins, then they’d also be hampered, and wouldn’t have much of an edge there. So it’s likely that ordinary fish don’t have these components, and this is exactly what (D) says, and the reason that (D) is the correct answer choice.
Now take a look at (A), which tells us that specially bred aquarium fish must receive special care if they are to survive. The word “must” makes this a pretty strongly worded answer choice, which we should be wary of in questions asking us for what’s most strongly supported. Yes, we are told that the specially bred fish are also underfed, again in the second sentence. But does that necessarily entail that they must receive special care if they are to survive? No. They could just be underfed and still survive, without needing any special care. Or they could be in tanks without regular fish. So we can’t really conclude that they require special care, which makes (A) wrong.
Hope this helps! Feel free to ask any other questions that you might have.