In literary terms, what is a foil?

A. The character who tries to prevent the main character from resolving the central conflict of the story.
B. A character who clashes with the main character in order to reveal the main character's traits
C. The character who is trying to achieve a particular purpose and who is usually the main character
D. A character who is usually a commoner and uses his or her wits to outdo people of higher social standing