Question

http://prntscr.com/9d3a80

Answer 1

Doubt is your key word here...doubt is basically a form of unsurety. Unsurety after a situation will lead to regret.
So C would be correct.

Answer 2

Yes

Answer 3

Please close this question so others know it has been answered, thank you! :)

Answer 4

A. Oh, I kept first for another day.
B. Yet knowing how way leads on to way.
C I doubted if I should ever come back.

I labeled them in an obvious more organized fashion, so C would be correct.