The paragraph talks about Tyrell's illness and his curiosity about the causes of this sickness. It also says that as a result of these discoveries, he became interested in "pathology". Based on the context, we would think that "pathology" should have something to do with disease or sicknesses and their root causes.
Look at the answer choices: we can definitely eliminate A and D because they don't pertain to disease at all. Now compare B and C. Well, B explicitly says "study of <em>disease</em>", which makes it the best candidate; "study of the body" is too vague, and it could talk about simply the workings of the body systems, rather than diseases.
