Explanation:

- Option 1: Rafael Nadal

- Correct Answer: Rafael Nadal won the 2019 US Open Men's Singles title. He defeated Daniil Medvedev in a thrilling five-set final.

- Option 2: Roger Federer

- Roger Federer is one of the greatest tennis players of all time, but he did not win the 2019 US Open. He was eliminated in the quarterfinals by Grigor Dimitrov.

- Option 3: Daniil Medvedev

- Daniil Medvedev was the runner-up in the 2019 US Open. He fought hard against Rafael Nadal in the final match but ultimately lost in an epic five-setter.

- Option 4: Novak Djokovic

- Novak Djokovic is another top tennis player, but he did not win the 2019 US Open. He retired from his Round of 16 match against Stan Wawrinka due to a shoulder injury.