Which of the following exercise program variables describes a change in intensity?

Switching from a treadmill to a stair-stepper represents a change in exercise mode or type, not a change in intensity.

Increasing the treadmill incline raises the grade of the surface, which directly increases the workload and oxygen cost of walking or running at the same speed. This constitutes a change in exercise intensity because the physiological demand (heart rate, energy expenditure, perceived exertion) increases without altering duration, frequency, or mode. Incline is a classic manipulation of intensity in cardiovascular exercise prescription.

Increasing the number of workout days from two to three per week is a change in training frequency, which is a separate exercise program variable from intensity.

Increasing running time from 20 to 30 minutes reflects a change in exercise duration or volume, not intensity, as the effort level per unit of time remains unchanged.