Since it’s an exterior class, I’ll assume that the context is crawlspace access.
Since it doesn’t specify whether the minimum is for access through a floor or through a perimeter wall, we’ll go with the smaller requirement of the two (i.e., the perimeter wall), which is 16x24 inches. That is per the IRC.
If you are taking a TREC exam. the TREC SOP does not specify a minimum crawlspace size, but only states that you are not required to enter a crawlspace or any other opening less than 18x24, but it never states that that is a minimum measurement or that you need to report as deficient an opening that is less.
Does that clarify things for you, considering neither of those answers are available for selection under the exam question?