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.

C. I was surprised by how loud my new baby sister could cry I might have to get ear plugs.

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.

it lacks a variety of sentence structure