Answer:
D. Esmerelda, a princess who secretly trains as a knight so she can prove
her courage
Explanation:
A round character is a complex personality that develops or morphs into something different as the story unfolds/develops. Such a character is easily identifiable with the audience as they can relate to him/her.
Therefore, in a story about Knights and dragons, the character that is most clearly round is option D. This is because, she is always changing and developing and isn't static.
