Fill in the blank with the correct word from the options given below:…

The correct answer is Option (1) → unless

Complete sentence: Unless the rain stops, the concert will have to be postponed. If the rain does not stop, the concert will have to be postponed, so "Unless the rain stops".

Option	Meaning and use	Example
(1) unless (answer)	Unless means "if not" or "except if". It introduces a condition.	You will miss the bus unless you hurry.
(2) until	Until means up to the time when and goes with an action that continues ("wait until the rain stops"). The sentence needs a condition (the concert is postponed only if the rain does not stop), not a time limit.	We stayed indoors until the storm passed.
(3) till	Till is an informal form of until and fails for the same reasons.	The shop is open till nine o'clock.
(4) while	While means during the time that something goes on. Stopping happens at a moment, so "while the rain stops" makes no sense.	She hummed a song while she cooked.