Choose the sentence that contains a comma splice. a) since it was chilly, i ordered some coffee at the snack bar. b) because this has been a very dry summer, the supply of reservoir water is very low. c) as i sat on a bench and drank my coffee, a seagull passed over, he relieved himself on my head. d) as i leaned over the railing of the ferry, i watched the seagulls dip down into the deep green water.

First of all, you need to know what a comma splice is. A comma splice, also known as a run-on sentence, means that two or more sentences are mixed into one large sentence but with improper punctuation. So, a comma is inserted where a period should be to separate two or more unrelated sentences.
Having this in mind, the correct answer is C. As I sat on a bench and drank my coffee, a seagull passed over, he relieved himself on my head. 
There should be a period between the words 'over' and 'he.'