C. Given that she had saved my life, I decided that some thanks were
in order
Explanation:
Understatements are when you write something as if it were less of a deal then it really is. In C, this person saved the narrator's life. That's a huge deal. Then the narrator said that "some thanks were in order" super casually, as if this woman didn't just prevent the narrator from death.
So, C is the most clear understatement out of all of the answers.
