Which of the following is a run-on sentence?
A. It was such a beautiful day, so we decided to eat lunch outside.
B. My favorite things to do on rainy days are to eat popcorn and watch movies.
C. I was surprised by how loud my new baby sister could cry I might have to get ear plugs.
D.The principal was concerned about the situation; he called a special meeting with the teachers.

Explanation: A run-on sentence is a sentence that conjoins thought with the usage of conjunctions and punctuation such as a comma or semi-colon. Choice C did not use any punctuation, besides a period (which doesn't matter in this case) and did not contain any conjunctions whatsoever.

Choice A is incorrect because the word "so" was used to bridge two thoughts. "So" is a conjunction. Also, a comma was used, so we know this isn't a run-on sentence.

Choice B is incorrect because the word "and" was used merge two thoughts. "And" is a conjunction.

Choice D is incorrect because a semi-colon was used to couple two thoughts into one sentence.

Thus, the answer is Choice C.