Choose the correct word from the given options, to fill in the blank…

The correct answer is Option (3) → until

Conjunction of time "until":

• Until means up to the time when; it marks the point at which a continuing action stops.
• Deciding clue: "stay" names a continuing action, and "you return" is its end point; staying here is a continuing action that goes on up to the moment of the other person's return.
• Complete sentence: I will stay here until you return.
  Example: Wait here until the bell rings.

Why the Other Options are Incorrect?

Option 1: when — Incorrect. When marks a point of time; "I will stay here when you return" says the staying begins at the return, not that it lasts up to it.
Example: Call me when you reach home.

Option 2: after — Incorrect. After would mean the staying begins only after the return, the opposite of waiting for it.
Example: We went out after the rain stopped.

Option 4: unless — Incorrect. Unless means "if not" and states a condition; the speaker would then stay only if the other person never came back, which is not the sense.
Example: You cannot enter unless you have a pass.

Contrast: "until" marks the end point of a continuing action; "when" marks a point of time, "after" a later start, and "unless" a condition.