Answer (A) is incorrect because it is off-topic. This question is asking you to pick the answer choice which allows the conclusion to be properly drawn. Essentially that means you are looking for the answer choice which fills in an assumption that the argument is making. Here, the conclusion is that all placentals in Australia are not native to the island continent because they could float/swim/etc. Answer choice (A) does not support this conclusion at all as it talks about marsupials, not placentals.
Answer (C) is correct because it fills in the assumption the argument is making. The argument is assuming that because all placentals in Australia had ancestors that can swim, fly or float, that means they must have used those means to get to Australia. It is not clear from the argument why this would be the case, so you need to pick an answer choice which forces that to be true. Answer (C) does this by telling us that the only way a placental would be native to Australia is if it's ancestors could not swim/fly/float. As that is not the case here, we know that the placentals can not be native. This answer choice is designed to throw you off as it's strangely worded, but in the end, is just filling a gap in the argument.
