With the given information the best answer seems to be C, for while wakes up does make sense with the wording "comes to" unless they woke up suddenly or to a surprise "with a start" does not fit. With the others options having no place the idea that someone "suddenly comprehends" fits being "comes to" equating to someone understanding or comprehending, and "with a start" meaning suddenly, for if something happens suddenly some people may be startled.
let me know if I need to delineate in more lucid terms.