Active immunity occurs when the person is exposed to a live pathogen, develops the disease and becomes immune as a result of the primary immune response.
When B cells and T cells are activated by a pathogen, memory B-cells, and T- cells develop, and the primary immune response results. Throughout the lifetime of an animal, these memory cells will “remember” each specific pathogen encountered, and are able to mount a strong secondary response if the pathogen is detected again.
So, the correct answer is option A.