It means conditions that are on the margin, the margin being that region of the operational envelope that is right on the edge between acceptable and unacceptable.
“The ship was as still as SHE could be” , they are personifying the ship as a she when it is not a living person
D. It makes april seem to the speaker like a mindless person to emphasize the contradictions of the month
The answer is C. All of the punctuation is correct in that sentence.