Which of the following sports is not part of the triathlon?

Horse-riding is not part of the triathlon because the triathlon is a multi-discipline endurance race that consists of swimming, cycling, and running. The sport requires athletes to compete in all three of these activities consecutively, testing their abilities in each discipline. Horse-riding, although another form of endurance sport, is not included in the triathlon as it focuses on equestrian skills rather than individual physical performance.