The most appropriate option for measuring the intensity level of any physical activity is "Rate of Perceived Exertion." This scale allows individuals to assess how hard they feel they are working based on their physical sensations, which can include feelings of effort, fatigue, and breathlessness.
The other options, "Length of workout" and "Speed Test," do not directly measure intensity; they may provide context about the duration or pace of activity but not the subjective experience of exertion.