The correct answer is (A): Both assertion (a) and reason (r) are true, and reason (r) correctly explains assertion (a).
Fever is an abnormal rise in body temperature above the normal range, which is usually a response to infection or inflammation. It is not a disease itself but a symptom or an adaptive mechanism of the body. Fever helps the immune system fight infections by creating an environment less favorable to pathogens and activating immune responses.