Final answer:
The Knickerbocker Rules, instrumental in forming the modern game of baseball, were introduced in 1845. They played a critical role in standardizing the game and were part of baseball's evolution into America's favorite pastime.
Step-by-step explanation:
The introduction of the Knickerbocker Rules, which laid the foundation for the modern game of baseball, took place in the year 1845. These rules were a significant advancement in the standardization of the game, contributing to its structure and play. As baseball developed, it gradually became America’s pastime, reflecting the country’s diversity, especially with the formation of leagues such as the Negro National League in 1920. Despite racial segregation in professional sports, baseball provided a platform for diverse communities and helped forge unified civic identities. Historic ballparks and the rise of salaried professional teams contributed to the proliferation of the game, which continued to grow in popularity through the 19th and 20th centuries.