Final answer:
A heart rate of 56 beats per minute in a sleeping 23-year-old athlete is considered abnormally slow, but not clinically significant if there are no other symptoms.
Step-by-step explanation:
Based on the information provided, a heart rate of 56 beats per minute in a sleeping 23-year-old athlete is considered abnormally slow. In a resting adult, a heart rate below 60 bpm is known as bradycardia. However, since the athlete is not exhibiting any other symptoms, such as weakness or chest discomfort, it is not considered clinically significant.
Therefore, the correct answer is C. Abnormally slow, and the nurse will recheck the heart rate before taking any action.