Oncology is the study of:

a. infectious diseases 

b. protozoan parasites

c. tumor

d. cancer

Oncology refers to the branch of science that deals with tumors and cancers. Onco means "bulk," "mass," or " tumor," and "-logy" signifies "study."
There are different types of cancer, but they all begin with the uncontrolled proliferation of abnormal cells. As a result, cancerous cells simply continue to divide, resulting in tumor-like aggregates of cells.