Which of the following best describes the type of training that consists of interval training on rough or natural terrain at various and random speeds and intensities?

Fartlek training is characterized by the incorporation of varied speeds and intensities during a workout, often in a natural setting. The term "fartlek" is Swedish for "speed play," which captures the essence of this training method. It blends continuous training with interval training, allowing the athlete to modify their pace and effort level throughout the session. This adaptability not only improves cardiovascular fitness but also enhances an athlete's ability to respond to changing terrains and conditions, making it particularly effective when performed on rough or natural surfaces.

In contrast, HIIT exercises feature structured intervals of intense effort followed by short recovery periods, often in a more controlled environment. Orienteering focuses more on navigation and physical endurance rather than specific training intervals. Hiking, while beneficial for fitness, does not generally involve the structured, varied pacing that is central to fartlek training. Therefore, the definition aligns most closely with how fartlek training is structured and executed in diverse environments.