The brief history on that subdomains question is that it was on an early practice AP test from the college board. It threw us (Code.org curriculum team) for a loop, too. To be honest, I'm not sure I even really know what they are. Subdomains are not mentioned in the framework explicitly. It was on an early practice exam and if I was a betting man, I would guess that when reviewed more in depth it's a question that might not make it to an actual exam because of the word "subdomain" not being terminology that's in the framework.
We left the question on our little practice test because seeing a question that you don't know the exact terminology for is very likely to happen on the real exam. But the concept of subdomains speaks to the notion of DNS being hierarchical which IS something students should have picked up, and looking at the question you can probably narrow down a few answers using intuition and what you know. Every end of unit test will likely have questions that stretch understanding beyond what was explicitly covered. Our hope is that using common sense and applying what you do know, you can come up with a plausible answer or at least narrow down from the answers given.