George offers to sell Penelope a ring that George found in his yard. Penelope tells George that she thinks it is a diamond, but George disagrees and thinks that it is probably just a shiny stone. Penelope pays George $10 for the ring. The ring turns out to be a diamond worth much more than $10. George wants the ring back, and Penelope refuses. What is the most likely result?

The ring will be returned to George because of equity.

The ring will be returned to George because of mutual mistake.

The ring will be returned to George because of unilateral mistake.

The ring will remain with Penelope.

The ring will remain with Penelope unless George can establish that she was negligent in not recognizing the ring's true value.