Which organization is classified as a national environmental group?

The Sierra Club is recognized as a national environmental organization because of its extensive reach and influence beyond local or state-level initiatives. Founded in 1892, it has grown into one of the largest and most prominent grassroots environmental organizations in the United States, with millions of members and supporters across all 50 states. The Sierra Club's mission focuses on promoting conservation, protecting wilderness, and advocating for sensible environmental policies at the national level.

In contrast, organizations like the California League of Conservation Voters and the Mendocino Environmental Center primarily focus on environmental advocacy within California, lacking the national scope that characterizes the Sierra Club. Friends of the LA River, while dedicated to a significant local initiative, is similarly concentrated on regional efforts rather than operating on a national platform. Hence, the Sierra Club stands out as the correct answer for its nationwide presence and influence in the environmental movement.