Answer: Option (b) is the correct answer.
Explanation:
When exercising for 20 minutes Molly's heart rate has not increased this means her body is not feeling tired or we can say that strength of her body is good.
For example, if a person does workout for about an hour daily then slowly and steadily his strength will increase
Therefore, even if Molly wants to improve her fitness level then she should increase the intensity of her exercise.