Which of these methods would be most effective in allowing someone to precisely gauge the intensity level of his exercise

You didn't provide us with any possible answers but if this is the list that you meant:
Complete the talk test
Ask a coach 
Take his pulse
Monitor his water intake

Then the correct answer would be C - take his pulse. 

Doing this, he would able to witness and explore what kind of stress did he put his body under.

20 times a minute is usually the average respritory rate of an adult. additionally the patient is breathing on his own, is clearly maintaining his airway, and is intaking a sufficient amount of air. you shouldn't use a bag valve mask, cpap, or other form of positive pressure ventilation, unless the patient has lost control of his airway, and is not breathing (apneic) or has rapid breathing or slow and shallow respirations. this patient would most likely and should be put on an non-rebreather mask or nrb attached to 100% high flow oxygen at 15 l/min. don't use a nasal cannula, as the amount of oxygen it provides is limited with a rate of 6 l/min.