The key in any science question is to try to cut through the jargon and get to the heart of the question. Take the word "autotroph". Auto, for our purposes in this question, means "able to do by itself." Think about an "automatic car that drives itself". Now lets go back to the question. You may not know what the phrase "troph" means, but you should be able to figure it out, or at least eliminate all the wrong answers. Hopefully you know that plants perform photosynthesis and thus were able to rule that out. Furthermore photosynthesis requires sunlight to excite the electrons in Photosystem II. There is not much sunlight in the deep ocean! Finally answers C and D can be ruled out because they require OTHER organism. Remember these "auto" bacteria do something by themselves right? The answer is B. Chemosynthesis is when an organism using methane as a source of energy. Its more complex than that, but you can research it if you have the time. The Bacteria do this on their own, and if you didn't figure it out, autotroph means that the organism is a producer that feeds itself. I hope that this answer was helpful.