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.

The answer is d because , safety almost always works, to prevent stds and pregnancy

The three things are: needs to be childproofmonitor if it can be dangerous to the babymonitor the baby's activity