Someone wants to dig for fossils. They go to school and specialize in geology. What subcategory would they be interested in?

That person would be interested in Paleontology

Explanation:

Paleontology is branch of Geology which seeks to get knowledge about how past organisms lived. For example, a subcategory of Paleontology is the Paleoecology which studies the relation of past organisms with their environments. Fossils can result not only from dead animals, plants can also produce fossils which can be leaf parts or even pollen grains. When pollen grains or leaf parts are found buried in the soil, you can infer past environmental conditions based on the taxa they belong to.