Heather wants to buy a new refrigerator, but she only has room for one that is 18 ft3 or less. which set of dimensions represent a refrigerator that heather can buy?

As soon as I read the words "which set" in your question, I knew that
there's a list of choices on the sheet where you copied the question
from, but that you didn't copy the list along with the question.

All I can tell you is this: 
Find a choice where, when you multiply all three dimensions together,
the product is 18 ft³ or less.  That choice is a set of dimensions that
represents a refrigerator that will fit in Heather's kitchen. 

(For all I know, there may be more than one suitable choice on the list that
you didn't include with the question, so you'd better check all of them.)