In strength training, a group of consecutive repetitions of the same exercise is called a set.
A repetition, or rep, is one complete performance of an exercise—for example, lowering and raising a weight once during a squat. A set is the block of repetitions performed together before the planned rest period. Thus, “three sets of ten” means completing ten repetitions, resting, and repeating that sequence two more times.
Sets organize a workout’s volume and intensity. The number of repetitions, the load used, the tempo, and the rest interval can all be adjusted to emphasize strength, muscle growth, muscular endurance, or general conditioning. A set can end before muscular failure, at failure, or after a prescribed time, depending on the program.
The other choices describe different training concepts. A drill is usually a practice activity, an interval is a timed work-or-rest period, and a circuit links several exercises in sequence. A circuit may contain multiple sets, but it is not the name for one consecutive group of repetitions.