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

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

Option 1 (unless) -> Creates a conditional: if the rain doesn't stop, the concert will be postponed - logical and correct.

Option 2 (until) -> Suggests the concert is postponed up to the point when rain stops - grammatically awkward in this context.

Option 3 (till) -> Same meaning as "until" - creates an awkward temporal relationship that doesn't fit the intended meaning.

Option 4 (while) -> Incorrect because "while" indicates simultaneous actions, but "stops" is not a continuous action.

Hence, Option 1: unless -> "Unless" establishes the correct conditional relationship, meaning "if the rain does not stop, then the concert will have to be postponed" - this perfectly expresses the intended cause-and-effect scenario -> correct