Which of the following describes a form of active immunity?

Define passive immunity: Passive immunity involves the transfer of preformed antibodies from another source, such as through the placenta, breast milk, or antiserum injections. This type of immunity does not involve the recipient's immune system actively producing antibodies.

Analyze the options provided: The first three options (transfer of antibodies across the placenta, injection of antiserum, and receiving antibodies through breast milk) describe forms of passive immunity because they involve the transfer of preformed antibodies. The last option, receiving a vaccine that stimulates antibody production, describes active immunity because the vaccine prompts the recipient's immune system to produce its own antibodies.