Final answer:
Active, previous exposure results in natural active immunity, where recovery from an illness like measles usually confers lifelong immunity against re-infection. The correct answer is option a).
Step-by-step explanation:
The correct answer to the question is a) Active, previous exposure. Active immunity refers to the resistance to a pathogen that occurs when an individual's immune system responds to a primary infection, leading to immunological memory. This type of immunity is naturally acquired when it happens after actual exposure to the disease-causing organism.
For example, individuals who recover from measles usually develop lifelong immunity to it and, hence, generally do not get infected again. Artificially acquired active immunity is obtained through immunization or vaccination, wherein a killed or weakened form of the pathogen or its components are introduced to the body to trigger an immune response without causing the full-blown disease.