The correct answer is D. To help reveal traits of the main character through their interactions
Step-by-step explanation:
In literature, the term "foil character" is used to refer to a type of characters that help the main character to develop either by indirectly supporting the character's actions or by showing opposite traits to the ones displayed by the main character, which helps the main character to stand out. Besides this, foil characters never have an important role in the story and do not solve the main conflict. According to this, the statement that best explains the role of a foil character is "To help reveal traits of the main character through their interactions" because the traits of foils are usually opposite and this helps reveal the particular traits of the main character.