If you show an adult a toy, and then hide it behind a curtain, the adult knows that the toy still exists. however, infants act as if a hidden object no longer exists. what does this exemplify?

The answer is object permanence.

Object permanence is the understanding that objects continue to exist even when they cannot be seen anymore. This is a fundamental concept studied in the field of developmental psychology. The term was coined by Jean Piaget. Jean Piaget’s theory is that children of about eight or nine months of age develop awareness/the idea that objects continue to exist even when one cannot see them. Before this stage, as far as the infant is concerned, items that are not within eyesight range do not exist.

answer; (d) one tectonic plate plunges beneath another; (most common,convergent boundaries)

the answer your looking for is c. : )