Elias conducted an experiment about the effects of exercise on the heart rate of teenagers. before morning basketball practice, he asked the members of his high school team to be his subjects. he measured each of their heart rates before and after a set of exercises. which change would most reduce the bias of his experimental design?

Well obviously he would need to make sure that the exercise is very well controlled.
But I think the most important aspect he needs to change is the people. Firstly, the subjects are all boys. They are also all obviously already quite fit (because they are in a sports team), and they are all around the same age. Results could vary if the sex, fitness and age of people also differs. It is therefore important to record results from a wide range of people.

The answer is option b.