Choose the appropriate word to fill in the blank in the given sentence:

_____ they had been waiting in line all night,I was sure they'd get the tickets for the show.

Since

The correct answer is Option (3) → Since

Since: This conjunction is used to mean "because" or "as a result of the fact that." It indicates a reason or cause. In this sentence, the reason for the speaker's certainty about getting tickets is the fact that they had been waiting all night.

Complete sentence: Since they had been waiting in line all night, I was sure they'd get the tickets for the show.