Answer:
Option C, Ocean
Step-by-step explanation:
An earth scientist study about the earth, its origin , its structure and components such as mountains, rivers, ocean, plateau & other geographical and topographical feature.
An earth scientist is not at all concerned with the study of animal cells, machine and farming methods. The studies related to an animal cell is carried by a biologist, studies related to machines is carried by mechanic /engineer and studies related to farming methods is carried by agriculturalist.
Hence, option C, oceans is correct.