"Is this a plain or a valley?"
Translation:Đây là một đồng bằng hay một thung lũng?
Obviously, both can be used in a question but there is a difference between Hay and hoặc :
"Hoặc is the stronger word for the absolute choice: either this or that.
Hoặc là tôi hoặc là cô ấy? Me or her? (An ultimatum)
That’s why hoặc is more used in legal texts, official documents and test (mathematical problems and questions).
Hay can be used in both formal in informal cases. There can be more options available- you can choose a third one or both."
PS : Đây là đồng bằng hoặc thung lũng? is accepted