The word "precursor" refers to something that comes before and serves as a sign or indication of what is to follow.

Similarly, the word "forerunner" means something or someone that comes before and indicates or precedes another thing or person.

In this context, the professor's thesis is being described as the initial or early indication or predecessor of nuclear studies.

Therefore, "forerunner" is the option that best captures the meaning of "precursor" in this sentence.