Which of the following describes the transfer of immunity from mother to fetus?

The transfer of immunity from mother to fetus is best described as passive immunity. This process occurs when a mother passes antibodies to her developing fetus during pregnancy, particularly through the placenta. These antibodies provide the fetus with temporary immunity against certain infections and diseases, which is crucial for the newborn's protection in the early stages of life.

Passive immunity is characterized by the transfer of antibodies rather than the body's own immune response developing its own antibodies. This is different from active immunity, where an individual's immune system actively produces antibodies in response to an infection or vaccination. Adaptive immunity, while related to the body’s ability to adapt and respond to specific pathogens, typically refers to the longer-term immune response and is not specifically tied to maternal transfer. Natural immunity usually refers to the immunity an organism is born with or gains through natural exposure to pathogens, rather than through maternal transfer.

Thus, passive immunity is the accurate description of the immune protection that a mother provides to her fetus, highlighting the mechanism through which antibodies circulating in the mother's body can confer protection before birth.