Which exercise variation focuses on developing the core?

The selection of planks or sit-ups as the exercise variation that focuses on developing the core is correct because both exercises specifically target the abdominal and stabilizing muscles crucial for core strength.

Planks engage multiple muscle groups simultaneously, including the rectus abdominis, obliques, and the transverse abdominis, while also enhancing shoulder stability and overall body alignment. Sit-ups primarily focus on the hip flexors and the abdominal muscles, promoting strength and endurance in the core region.

In contrast, push-ups and pull-ups are primarily upper body exercises, targeting the chest, shoulders, and back muscles, with secondary core engagement. Lunges and squats are lower body exercises concentrating on the legs and glutes, while also requiring some core stability but not primarily targeting the core. Lastly, cardio bursts or running focus on cardiovascular fitness and endurance rather than specifically strengthening the core muscles. Thus, planks and sit-ups are uniquely aligned with core development, making them the correct choice.