Part A: Which bullet point from the brochure does the author expand upon in the section titled "How can endorphins be dangerous in
sports?"

The correct answer to this open question is the following.

Unfortunately, you did not include the bullet points or the brochure. However, we can comment on general terms.

The body produces neurochemicals as a result of stress and even pain. The pituitary gland produces this substance called endorphins. What this substance do is to alleviate most of the pain or stress produced by stressful activities, tense moments, or the daily office routine.  

So exercise is good for the body up to the degree of no forcing it beyond its natural capabilities. This means that you exercise to be healthy and feel good, but never force it to cause you stress or pain, trying to accomplish something that is out of reach. So the doctor's recommendation is to never put so much stress on your body. A common routine of 45 minutes yo one hour, is what your body really needs.  

alone =having no one else present.

i feel like it is actually b.