Explanation: Active immunity is acquired when the body's immune system is exposed to a specific pathogen or antigen, either through vaccination or natural infection. In response, the immune system produces its antibodies to protect against future encounters with the same pathogen.

Incorrect choices:

a. Innate immunity is the first line of defense against infections and is present at birth. It includes physical barriers, such as the skin and mucous membranes, and non-specific immune responses that do not require prior exposure to a specific pathogen.

c. Passive immunity is the transfer of pre-formed antibodies from one individual to another, providing immediate, temporary protection. This can occur naturally, such as the transfer of antibodies from a mother to her baby during breastfeeding, or through the administration of pre-formed antibodies, as in certain medical treatments.

d. Natural immunity refers to the immunity that is acquired through natural exposure to a pathogen and the subsequent development of an immune response. This includes both active immunity from natural infection and passive immunity from maternal antibodies passed to the baby during childbirth or breastfeeding.