Ellison exercised a nonstatutory stock option on May $31,2023.$ He paid $$16,000$ for $100$ shares of his company's stock. The fair market value of the shares on that date was $$20,000.$ Although Ellison continues to hold these shares, he included the $$4,000$ difference between the fair market value and the purchase price as compensation income when he filed his $2023$ return. What is Ellison's basis in the stock?

$$0$

$$4,000$

$$16,000$

$$20,000$