Which of the following is an example of acquired immunity?

Acquired immunity refers to the protection that develops through exposure to specific pathogens, typically through either infection or vaccination. The correct answer, which involves desensitization through vaccination, exemplifies this type of immunity because vaccines introduce a harmless part or a weakened form of the pathogen, prompting the immune system to develop a response. This response includes the creation of specific antibodies and memory cells that provide long-lasting immunity against that pathogen.

The other choices do not illustrate acquired immunity. Exposure to a pathogen could lead to the development of acquired immunity, but it does not specifically define the process like vaccination does. Natural killer cell activation is a part of the innate immune response and does not pertain to the specific, learned responses characteristic of acquired immunity. Finally, antibodies inherited from the mother represent passive immunity, providing protection to the newborn through maternal antibodies rather than the individual's own immune response.