Answer:
C) Of course, there was always a chance he knew the suspect all along.
Step-by-step explanation:
Transitional words express relationships between sentences and paragraphs. In that respect, he transitional word of course introduces an assumption or synopsis, so it must be separated from the rest of the sentence by a comma.
The rest of the sentences are punctuated incorrectly because they either do not separate the transition word from the rest of the sentence, or they have commas in the middle of the sentences, sometimes separating the subject from the verb.