Answer:
B. A character who clashes with the protagonist but is on the same side
Step-by-step explanation:
A foil is actually a character that has core values and characteristics that oppose to a character whom with the foil is supposed to clash, making the that character stand out. If the main character is selfless, the foil character will be selfish, for example, to make the main character's selfless stand out. The foil can be on the same side of the character, as long as he has contrasting characteristics or values.