Which of the following is classified as a strength-training activity?

Sit-ups are classified as a strength-training activity because they specifically target the core muscles, including the abdominal muscles. Strength training involves performing exercises that cause the muscles to work against a resistance, leading to muscle strength and endurance development. Sit-ups require the body to lift and lower the torso, engaging various muscle groups and promoting muscle growth and stability.

In contrast, the other activities listed do not primarily focus on strength training. Tennis, while an excellent cardiovascular workout that promotes agility and coordination, mainly relies on muscular endurance and speed rather than strength development. Jump roping is predominantly a cardiovascular exercise that improves stamina and coordination but does not significantly contribute to building muscular strength. Hopscotch focuses more on agility and balance, engaging different muscle groups but lacking the resistance aspect that characterizes strength training.