Answer:
Special donor
Explanation:
In the given case, George Soros pledged to contribute $3 million to the center. Here, it must be understood that the Center for American Progress is a progressive public policy research and advocacy organization and therefore, has a specific function that the center perform or contribute towards. When the contribution is pledged for a purpose or is restricted to be used for specific function or in this case for a specific center for policy research, then the person giving the contribution will be considered a special donor.