The correct answer is
C: The Erdős–Bacon number is a variation on the “Six Degrees of Kevin Bacon” phenomenon, where actors determine how many levels of separation they have from Kevin Bacon in their performing roles. The Erdős–Bacon number combines the degrees of separation of an actor to Kevin Bacon along with that same person’s connection to mathematician Paul Erdős, counting degrees of separation in co-authoring academic papers. Portman has a Bacon number of 2 and an Erdős number of 5, for a combined Erdős–Bacon number of 7, as do actors Colin Firth and Kristen Stewart. Danica McKellar (Winnie Cooper on The Wonder Years) has an Erdős–Bacon number of 6.