The Colville Indian Reservation makes up part of the population of the Okanogan Highlands region. The correct answer is option D.
The Colville Indian Reservation contributes to the population of the Okanogan Highlands region. Nestled in the northeastern part of Washington state, this reservation encompasses diverse landscapes, including valleys and mountains. Rich in cultural significance, it is home to the Colville Confederated Tribes, playing a vital role in preserving their heritage.
The Okanogan Highlands region, characterized by its scenic beauty and natural resources, reflects the intertwined relationship between the reservation and the broader geographical context. The reservation's presence underscores the significance of indigenous communities in shaping the cultural and environmental fabric of the Okanogan Highlands.
Option D is the correct answer.