Either investigates or elaborates. I think it's elaborates
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.)
C) Alienate
All the other choices don't make sense...
Supposed*
You had a grammatical error and I had to fix it considering you’re an English teacher.
I could see that my fire maker was having a hard time. But I gave him his space, because I wanted the warmth of the fire.