The course has the wrong answer listed as correct in unit 7.5 exercises.
These two words have the same meaning and similar usage to each other except for:
면 / 이면 (myun/imyun)
와 / 과 (wa/gwa)
그러나 / 하지만 (geuruna/hajiman) both mean "but"
나 / 구나 (na / guna) these 2 do not mean the same -- 구나 is an exclamatory ending nor related to "나" which is usually used to mean "or".