FWIW, I've more commonly heard this form for asking directions: 去机场怎么走？
and it was marked correct.
I would have written
if 到 was an option. Why isn't it needed here?
And which question would sound the most natural?
I think with 到 sounds much more natural than without
How about 请问飞机怎么去 this one i think is correct.
飞机 is airplane, not airport
Add a 机场 and I agree (but Duolingo didn't give me credit for that either).