The correct answer is c: >In class E airspace when IMC exists or in class A airspace.

This weeks was a bit of a gimme… A simple process of elimination lets us get rid of option a, since we know that class G airspace is “non controlled” and therefore does not need a flight plan. The same goes for option b, as defense zone airspace does not require an IFR flight plan if weather conditions allow VFR flight.

Check out 14 CFR 91.173 and 91.135 for the IFR flight plan requirements. As always, the AIM is also a good place to look for more information, too.

The regulations are sometimes (often) written such that you need to think “in reverse…” Here, they say “no person may” operate under instrument flight rules unless… Or “no person may” fly in classA airspace unless… And if you know these two sections, even without the process of elimination above, answer c becomes the only fully correct answer.