The question is definitely hampered by its own inconsistent logic. The first sentence states that all seniors take both calculus and physics. The second sentence contradicts the first: Rocky is a senior and yet he is not taking physics. We conclude that the question writer meant calculus OR physics. If that conclusion is correct then (a) is the correct answer because if Rocky, a senior, is not taking calculus, then he must be taking physics. (In symbolic form, C OR P means not C --> P and not P --> C.) Continuing on the conclusion that the writer meant C OR P rather than C AND P , we can't be sure that II is correct. Marsha is a senior who may be taking calculus, but she may have opted to take only physics and we don't know for sure. III is not necessarily correct, because the given premises do not say that ONLY seniors may take calculus. In this example John may be a junior or sophomore who is taking calculus.
If the writer did mean to say that all students do in fact take both calculus and physics, then the statement about Rocky being a senior and not taking physics is false. This ruins the question, but looking at the 3 choices of things that MUST be true... I is difficult to decide since Rocky seems to be lying about either his schedule or his class. I suggest we detain him at Guantanamo Bay. II is correct, because Marsha is a senior and therefore she must be taking both calculus and physics. III is still not necessarily true since the same possibility that John is an underclassman enrolled in physics remains open.
