Which author is primarily known for writing contemporary realistic fiction?

The author primarily known for writing contemporary realistic fiction is Ann Martin. She is best recognized for her popular series, "The Babysitter's Club," which portrays the everyday lives, challenges, and friendships of a group of young girls dealing with relatable issues. This type of fiction reflects the social realities of adolescents and allows readers to connect deeply with the characters and their experiences.

In contrast, the other authors listed are associated with different genres. J.K. Rowling is widely known for her fantasy series "Harry Potter," which is set in a magical world. Roald Dahl's work primarily falls within children's literature that often includes fantastical and whimsical elements, such as "Charlie and the Chocolate Factory" and "Matilda." C.S. Lewis is mainly recognized for his fantasy works, particularly "The Chronicles of Narnia," which involve adventures in a fictional universe. Each of these authors has a unique style and thematic focus, making Ann Martin's contribution to contemporary realistic fiction distinct and significant.