Which Is the Best Example of Underextension

Multiple Choice

Which is the best example of underextension?

A) Referring to a train as a "choo-choo"
B) Referring to all vehicles having four wheels as "trucks"
C) Using a single-word utterance ("car") rather than telegraphic speech ("let's ride in the car")
D) Referring to the family cat as a "kitty" but not using the same name for any other animal (including other cats)