The treatment of diseases by using drugs is known as

Given data: It is asked in the question what term is used to describe the treatment of diseases by the use of drugs.

Concept Used:  Definition of Chemotherapy

Detailed Solution:

Chemotherapy is a pharmacological therapy that targets your body's rapidly proliferating cells with potent chemicals or drugs. 

Since cancer cells grow and proliferate significantly more quickly than the majority of body cells, chemotherapy is most frequently used to treat cancer.

Hence, the required option is C).