Final answer:
The simplest way of defining intensity in an erg workout is by increasing the heart rate (Option C).
Step-by-step explanation:
Heart rate is a measure of how hard your heart is working during exercise, and it can be used as an indicator of workout intensity. When you exercise at a higher intensity, your heart rate increases, indicating that you are working harder and burning more calories.
While the other options - calories burned, distance covered, and rest intervals - can also provide information about the workout, they are not as direct and specific in measuring intensity as heart rate. Heart rate can be easily monitored using a heart rate monitor or by taking your pulse during exercise.
Therefore, the correct answer is C) Heart rate.