Answer:
C. Add a period after fossil
Step-by-step explanation:
The reader can easily deduce that this is a run-on sentence.
"Look at this fossil," Is a full stop on itself, and the thought can end there.
The next phrase can be continued in a separate sentence by itself.
Many people seem to be confused as to why the answer isn't B, and I'll gladly explain why!
The sentence goes, "Look at this fossil I bought it at the gift shop.."
"It," causes the sentence to become a run-on, and therefore isn't grammatically correct.
However, if the sentence said, "Look at this fossil I bought at the gift shop," then it would have no need to be changed, as it is a full sentence in itself with no run-on.
Another thing to note is that some of you may be thinking, "But (insert grammar checker) said it was fine by itself!"
I'm sorry to break it to you, but a grammar checker doesn't have 100% guaranteed results. Many grammar checkers do not possess the ability to catch run-on sentences and are not a credible source of information.
I hope this clears things up!