In an outline capital letters can indicate many different things, but almost always they are used to signal a change in topic, or at lease a change in subtopic.
The answer is B because it puts a coma in the correct spot to pause between the first half of the sentence and the second. The others either randomly placed a period, using the wrong word choices, or made it a run on sentence. :)
He glanced apprehensively at him, apprehensively
