Question

the performance of the heart is closely monitored by what system of the body?

Explanation:

The nervous system, specifically the autonomic nervous system (with its sympathetic and parasympathetic divisions), regulates and monitors the heart's performance. It controls heart rate, contractility, etc. The cardiovascular system is the heart itself along with blood vessels. The muscular system is about muscles in general (not specific to heart monitoring). The respiratory system deals with breathing.

Answer:

Nervous system