Final answer:
The nurse's best response would be to advise the patient to slow down and aim for an exertion level below 12.
Step-by-step explanation:
The nurse's best response would be: 'Slow down a bit; ideally you should be less than 12.'
Based on the patient's rating of 15 on a scale of 1 to 20, it indicates that the patient is exerting herself at a high level. In heart failure, it is important to avoid excessive exertion to prevent worsening of symptoms and complications. The nurse's response suggests that the patient should slow down and aim for a lower exertion level, ideally below 12, to ensure safety and promote recovery.