What cavalry regiment — known as the “Rough Riders” — did Teddy Roosevelt join during the Spanish-American War?

What cavalry regiment — known as the “Rough Riders” — did Teddy Roosevelt join during the Spanish-American War?

1st U.S. Volunteer Cavalry Regiment
2nd U.S. Volunteer Cavalry Regiment
3rd U.S. Volunteer Cavalry Regiment
4th U.S. Volunteer Cavalry Regiment
5th U.S. Volunteer Cavalry Regiment

The correct answer to this quiz is 1st U.S. Volunteer Cavalry Regiment.