Final answer:
An extremely important resource for immigrants transitioning to life in America was family and community support, which provided a familiar and supportive environment amidst the challenges of acclimation.
Step-by-step explanation:
For immigrants, an extremely important resource in the transition to coming to America was A) Family and community support. During periods of migration to the United States, including between 1870 and 1900 when around 12 million immigrants arrived, many of them faced challenges such as learning a new language, adapting to new customs, and finding employment. These difficulties were often eased by settling into ethnic enclaves, where immigrants could rely on the support of a community that shared their language, cultural heritage, and familiar social practices. Living within their own immigrant communities also provided them with a sense of security and comfort, maintaining their cultural identity amidst the backdrop of America's diverse society.