Bëor the Old is a fictional character in J.R.R. Tolkien's Middle-earth legendarium. He appears in "The Silmarillion" as the leader of the First House of the Edain in the First Age, which was called the "Folk of Bëor" after him. He was the father of Baran and Belen and ancestor of Beren Camlost.
Beren is Bëor's favorite child he fathered.
It's incorrect because Beren is not one of Bëor's children. Perhaps it was stumped because I worded it a little unusually to make sure it was clear that I was intending to class Beren as Bëor's child, not just someone else's child Bëor liked.