Answer:
C. There is no error.
Step-by-step explanation:
A. is incorrect because it contains a comma splice which is the use of a comma to join two independent clauses: "I saw the accident, I was asked to testify."
B. is incorrect because it contains a fragment: "Because I saw the accident" does not contain even one independent clause and cannot stand by itself as a sentence.
C. is correct because the sentence in question is a compound-complex sentence as it contains two independent clauses and one dependent clause and uses punctuation correctly.