Final answer:
The nurse's best response would be to acknowledge the patient's observed behaviors and explore further to understand their experience.
Step-by-step explanation:
The nurse's best response to these observations would be to acknowledge the observed behaviors and explore further to understand the patient's experience. The patient's non-verbal cues of rapid breathing, clenched fists, and facial grimacing suggest that they may be experiencing pain, despite denying it verbally. By acknowledging these behaviors, the nurse can show empathy and concern, and further investigate the patient's condition to provide appropriate care.