The correct answer is (A): Both assertion (a) and reason (r) are true, and reason (r) correctly explains assertion (a).
Fever, an elevated body temperature exceeding the normal range, typically signifies an immune response to infection or inflammation. It functions as a symptom and an adaptive bodily mechanism rather than a disease. Fever aids the immune system in combating infections by fostering an environment hostile to pathogens and by stimulating immune responses.