Final answer:
a) a critical period. The period of development in which a teratogen can exert its effect is called a critical period. It is a crucial time for prenatal development.
Step-by-step explanation:
The period of development in which a teratogen can exert its effect is called a critical period. During this time, specific parts or organs of the fetus develop, and exposure to teratogens can permanently deform the structure or function of the developing embryo or fetus. For example, alcohol exposure limited to a specific day during gestation can lead to significant facial abnormalities in the offspring. The critical period is a crucial time for prenatal development, and avoiding exposure to teratogens during this period is critical for the health of the developing baby.