The correct answer is B) mixing into a culture.
The choice that best defines the underlined word in the following sentence: “The American government was concerned about the assimilation of Native American peoples," is "mixing into a culture."
The underlined word is "Assimilation." In the context of the statement, it means to absorb the tradition, culture, and customs of a specific group. In this case, the US government was concerned about the way to absorb, to mix or intermingle the Native American Indian Tribes traditions with that of the white Americans.