Fill in the blanks with the most appropriate option:-

You are supposed to return to the base camp ……….. three days.

within

The correct option to fill in the blanks is within.

The sentence is about a deadline for returning to the base camp, and the word "within" is the most appropriate word to use in this context, as it means "in or during a specified period of time".

The other options are not as appropriate:

• After means "later than". It would not be accurate to use "after" in this context, as the speaker is not saying that the person must return to the base camp after three days.
• Until means "up to the time that". It would not be accurate to use "until" in this context, as the speaker is not saying that the person must return to the base camp up to three days.
• Before means "earlier than". It would not be accurate to use "before" in this context, as the speaker is not saying that the person must return to the base camp before three days.