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

_____ the rain stops, the concert will have to be postponed.

unless

The correct answer is Option (1) → unless

Unless is a conjunction meaning "except if" or "if not." It introduces the condition that must be met to avoid the consequence.

• Meaning: If the rain does not stop, the concert will have to be postponed.

• until / till → talk about time, not conditions.
  Using them would incorrectly imply the postponement itself lasts until the rain stops.

• while → means “during the time that,” which changes the meaning entirely.