Which of the following is an example of naturally acquired active immunity?

Naturally acquired active immunity occurs when an individual's immune system is exposed to a pathogen and subsequently generates its own immune response to that pathogen. This process involves the body recognizing antigens from the infectious agent, like a virus or bacterium, leading to the activation of B cells and T cells to produce specific antibodies and memory cells for long-term immunity.

In the case of a diphtheria infection, the body encounters the diphtheria bacteria (Corynebacterium diphtheriae) and mounts an immune response against it. The individual becomes sick during this natural infection, but as a result, the immune system creates antibodies specific to the diphtheria pathogen. This helps the person develop immunity against future infections by the same bacterium.

Other options provide a contrast to naturally acquired active immunity. Vaccination is an example of artificially acquired active immunity because it introduces a harmless form of the pathogen or its antigens to stimulate an immune response without causing the disease. Exposure to toxins does not directly stimulate the immune system in the same way. Antibody transfer from mother to child represents passive immunity, as the child receives pre-formed antibodies rather than generating its own immune response.