Which system must include a nurse call system as part of its critical branch?

The essential electrical system is the correct choice because it is designed to provide power to critical areas that support life safety and health care operations during a power outage. This typically includes systems essential for patient care, such as lighting, communications, and alarms.

A nurse call system is vital in healthcare environments for patient monitoring and emergency response. Integrating the nurse call system into the essential electrical system ensures that it remains operational even during electrical failures or outages, allowing healthcare providers to respond quickly to patient needs, which is critical in a medical setting.

The other options, while important for various functions, do not necessarily mandate the inclusion of a nurse call system as part of their core design. Backup power systems may provide power to various equipment but do not specifically address patient communication. Standard lighting systems generally focus on general illumination rather than critical communication needs. Emergency service systems may pertain to external emergency responses and not internal patient care operations. Thus, the essential electrical system stands out for its role in maintaining critical communication like the nurse call system.