This seems to be a declarative sentence. It talks about the reaction of the policeman to a specific thing. His expression is not pleasant. What he might saw is something not good and something to worry about. Other possible thing could be there an heinous crime scene that he faced at that moment. It depends on the situation.<span />
A run-on sentence is a sentence having two independent clauses with improper punctuation and without conjunctions. The correct answer is option b. It is difficult to understand a run-on sentence because you cannot identify where to pause or stop and that the sentence does not have conjunctions which would help in the transition of clauses.
