Answer:
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The answer is c. He was tired and it was late; it had been a long day. The semi-colon helps distinguish the two separate parts. The part where it says "it had been a long day" doesn't make sense if added by a comma.
It is a run-on sentence. You have a complete sentence (verb and a noun), I like chicken, and then you have another complete sentence, I hate broccoli. If you had put an and between these two sentences you would have had a compound sentence. Hopefully this helps.
