Questions & Answers about De må gjette svaret på utslagsspørsmålet.
Here De means formal you, not they.
In Norwegian, lowercase de normally means they, but capitalized De is the old-fashioned or very formal polite form of you. It can refer to one person or more than one person, so context tells you whether it is singular or plural.
So in this sentence, De is a polite way of addressing someone directly.
Not very common in everyday speech.
Most modern Norwegian speakers would usually say:
Du må gjette svaret på utslagsspørsmålet.
if speaking to one person informally, or
Dere må gjette svaret på utslagsspørsmålet.
if speaking to more than one person.
Using De today often sounds very formal, distant, old-fashioned, or deliberately polite. You may still see it in older texts, in some formal service language, or in situations where someone wants to sound especially courteous.
Because må is a modal verb, and modal verbs in Norwegian are followed by the bare infinitive, without å.
So you say:
De må gjette
not
De må å gjette
This is the same pattern you get with other modal verbs:
kan gjette = can guess
skal gjette = shall / will guess
vil gjette = want to guess
bør gjette = should guess