I'm not the greatest at grammar, but I think the very first one is incorrect. It should just be a comma, then the second just a comma as well, but the last one is correct. (I have no idea, I am sorry if you get it wrong.)
The best answer is the first option:
<span>For seven nights, he sneaks into the old man’s room and watches the sleeping man.
A good way to select the right answer is to read all of the options out loud. Which combination of the sentences sounds the most fluid and logical when you read it, and best preserves the meaning of the sentence? The first option does this best. </span>
Diction
I think diction has to do the most with the subject of word choice
