Final answer:
Baseball (option D) became the national pastime in the late 1800s, gaining widespread popularity through professional teams, the formation of the National League, and cultural icons like Babe Ruth.
Step-by-step explanation:
The sport that became the national pastime in the late 1800s is D) Baseball. During this era, professional baseball teams had begun to emerge, providing affordable entertainment and aiding in the creation of a unified civic identity amongst the diverse American population. The National League was established by 1876, and iconic ballparks that played a significant role in community gathering and identity were constructed. Athletes like Babe Ruth also played a critical role in popularizing baseball, becoming symbols of American culture. By contrast, while sports like boxing, football, and others garnered popularity, they did not achieve the widespread cultural integration and emblematic status that baseball held during this period.