Answer:
Repentant: feeling remorse to a degree marked by an extreme change.
Explanation:
The word extreme is a keyword here - it relates to very powerful and strong emotions caused by some kind of a change. Usually, something awful happens, or you do something really negative that affects a lot of other people, and then you feel repentant and wish that hadn't happened at all or that you could change the situation or your actions. The other options do not exhibit the same degree of regret as repentant does.
