Which is a run-on sentence? The sun was setting in the west, the clouds had become a vibrant pink.

My alarm jolted me awake, and I sprang out of bed.

Unless we distribute these flyers, people won't know about our concert.

(PICK ONE!)

Number 1

Explanation:

The sun was setting in the west, the clouds had become a vibrant pink. There is no form of transition in this sentence unlike the other 2. Sentence 2 uses "and" to separate two things that happened in the sentence and Sentence 3 uses a comma before introducing another thing in the sentence.

The first sentence doesn't use any word or comma to separate the two things within the sentence so it's a run-on sentence.