Final answer:
Regionalists, master realists, and naturalists were groups of writers that significantly influenced American literature by focusing on detailed depictions of contemporary life, character psychology, and human behavior as influenced by heredity and environment.
Step-by-step explanation:
The groups of writers known as regionalists, master realists, and naturalists each contributed uniquely to American literature towards the end of the 19th century. Regionalists added depth to Local Color by creating more three-dimensional characters and a protagonist's point of view, often with an element of nostalgia or sentimentality. Master realists like Mark Twain, William Dean Howells, and Henry James focused on the accuracy of detail and contemporary life, avoiding authorial intrusion and sentimentalism. Naturalists, influenced by Charles Darwin and Emile Zola, pushed realism further by exploring human beings as products of heredity and environment, tackling controversial subjects like prostitution and domestic violence with a scientific observation approach.