I believe that it is because it is "können" in this example, and not "konnten" (Präteritum) or "könnten" (Konjunktiv II - Präteritum), or some other form of können.
And "könnten Sie" can be considered the (even more) polite form of "können Sie" (which is the 'formal you' form of "kannst du").
Therefore, it is usually advisable to translate "können" to "can" and "könnten" to "could".
No, because "Sie" is capital here, indicating that it is the polite form of you.
"Could you?" sounds more polite than "can you?". But "could" was not accepted - is that wrong?
"Could you?" = "Könnten Sie?"
"Könnten Sie?" is the more polite version of the German sentence given here. So they are not the same and therefore not interchangeable. That's why you have to stick to the "Can you?"-translation here.
I tried answering: Can they?
Is there any way for me to know it should be 'Can she?'
"Sie" (with a capital S) is formal "You". "You all" is "Ihr könnt/könnte (I don't know exactly).
The word for all was not in the sentence. It would be used at one person not many.
The "Sie" is capitalized in the German sentence although it is not the first word of the sentence. So it has to be the formal you!
"Können SIe?" = Can you I typed "Could you?" and got marked wrong. Surely this should have been accepted