Final answer:
The heart rate reserve is the difference between maximum heart rate and resting heart rate, which in this case is 136 bpm (208 - 72), corresponding to option D. 136.
Step-by-step explanation:
Your heart rate reserve is the difference between your maximum heart rate and your resting heart rate. If you have a resting heart rate of 72 and a maximum heart rate of 208, you can calculate your heart rate reserve by subtracting your resting heart rate from your maximum heart rate.
Heart Rate Reserve = Maximum Heart Rate - Resting Heart Rate
Heart Rate Reserve = 208 beats per minute (bpm) - 72 bpm = 136 bpm
So, the heart rate reserve would be 136 bpm, which is option D. 136.