What is the definition of chemotherapy?

Chemotherapy is defined as a treatment for disease, primarily cancer, that utilizes chemical substances to target and kill rapidly dividing cells, which characterizes cancerous tissues. This process often involves the administration of various drugs that can inhibit the growth of cancer cells or induce their death. The use of chemicals in this context is crucial, as these agents can circulate throughout the body, allowing them to reach cancer cells that may have spread beyond the primary tumor site.

While other options involve methods associated with cancer treatment, they do not fit the definition of chemotherapy. Surgical procedures focus on physically removing tumors, immunotherapy employs the body's own immune system to fight cancer, and diagnostic techniques involve imaging to identify health conditions rather than treating them. Therefore, option A is the most accurate representation of chemotherapy.