The ability to learn new words after being exposed to it once is
a. overregularization
b. telegraphic speech
c. underextension
d. fast mapping

The answer is D. The ability to learn new words after being exposed to it once is called fast mapping.

Allthough all of the options correspond to different stages in language adquisition and development, fast mapping is the only one that describes the process in which a child learns a new word by contrasting it with another word that is already familiar to him. Once the contrast is made, the unknown word will be remembered by the child a week later.