C is technically true; however, it doesn't actually tie in with the process the article describes. In the article, the focus is on the
difference between temperatures at different depths in the ocean, not the simple presence of heat energy. To run through the answers one by one:
A is fairly obviously incorrect. While it's true, it has nothing to do with this article. The energy produced by the oceans is largely mechanical, while the text focuses on heat.
B is tempting. It discusses the difference in temperatures that the article goes into so much detail on. A deep reading can catch this one, though - when it describes the process, the article talks about "cold water pumped up from the ocean depths," and this answer says that deep water being cold is a problem. So, incorrect, but it's one of the "trick" answers.
C is like A: true but irrelevant. The point of the article is not that the oceans have a large amount of energy stored away but rather that there are a couple of methods for getting at that energy. Both of those methods use a temperature differential to run the plants, not just raw energy from the water, so C doesn't actually say anything meaningful. Yes, it's almost the same as one of the first sentences in the article, but that doesn't make it the main "factor that can be used as an energy source." That's the difference, not the simple amount.
D is the only one left, but it's good to look over it first. The article never mentions tropical regions at all, but it
constantly mentions warm shallow water. This question expects you to use some outside knowledge, specifically that shallow water tends to stay close to the local temperature while deep water is almost always cold. Since the plants need warm water at the surface and cold water below, that makes tropical areas the best place to build them. Again, it's not written in the article anywhere, but that's why the question asks you to "infer."
This was definitely a tricky question considering it had two very tempting wrong answers - and, for that matter, that all three wrong answers were still true to some degree. Be careful not to simply select the answer that seems to be
written in the text, because that's frequently bait to stop you from seeing the actual focus.
I hope this helps!