The crrect nswer is C. The pursuit of knowledge
The advisor is giving a speech about how to become a "man of science," and s/he is arguing that in order to become one, one should study every branch of natural philosophy and not just one specialty. Natural philosophy was a term used during the Middle Ages and the Renaissance to refer to Natural Sciences before science emerged as an institutionalized and rational method of studying the reality. The advisor also contrasts the concept of "man of science" with "petty experimentalist," which is that that one would become if you do not apply yourself to the study of every branch of science. Here, the speaker is stressing the importance of learning about different fields of study, in order to think out of the box and make connections between different topics that, other way, could not be made. This is the main difference between a man of science, who searches for true knowledge, and petty experimentalists, who just apply themselves to a particular field of study.