Question

the word in bold is vague. select all the words it could refer to.
donatello invented a new dance and taught it to his grandma. that may not have been a great idea.

Explanation:

The bold word "That" refers to the entire prior action described: inventing a new dance and teaching it to his grandma. This action is the "idea" referenced, so the phrase it points to is the full preceding activity.

Answer:

Donatello invented a new dance and taught it to his grandma