Final answer:
An earth scientist is involved in the study of different elements that compose the Earth's systems, including oceans, which is a field known as oceanography.
Step-by-step explanation:
An earth scientist studies various aspects of the Earth, including its physical constituents, processes, and history. Subjects that an earth scientist might study include the shape and motion of physical objects, the formation of metamorphic rocks, and galaxy formation and evolution. When it comes to the provided options for what an earth scientist studies, the closest match would be 'C. oceans,' as Earth scientists could dive into oceanography, which deals with all aspects of the ocean's physical and biological systems.