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The largest part is syndication fees. You're seeing a pie chart about public radio station income, not NPR's income. See further down on the same page, in which it is indicated that 37% of unrestricted NPR income is from program syndication fees.

"Program fees and dues paid by our Member Stations are the largest portion of NPR's revenue. This includes fees paid to air the NPR newsmagazines, other programming we produce and distribute and annual member dues." and