Underextension in language occurs when a child incorrectly uses one word to describe other objects or actions that are similar to that word.

Actually, that's over-extension. Under-extension occurs when a child uses a word that can describe multiple things in an exclusive manner.

Ie - "Timmy is my friend".

"Is Andy your friend?"

"No. Timmy is my friend!"

"How many friends do you have?"

"Timmy".

The child is under-extending the word friend because, he thinks that only Timmy can be 'his friend'.

The answer is False.