The protocol described is Modbus, which is a commonly used industrial protocol that lacks built-in authentication and security features. Modbus operates in a client/server model and can be implemented over RTU (Remote Terminal Unit) or TCP/IP for communication between devices. The other protocols mentioned either have different characteristics or are used in different contexts (such as Profinet for industrial automation, Zigbee for wireless IoT devices, and Z-Wave for home automation). CASP+ identifies Modbus as a critical protocol in industrial environments that lacks security and requires additional protective measures. Reference: CASP+ CAS-004 Exam Objectives: Domain 4.0 - Industrial Control Systems (ICS) and Modbus Protocol CompTIA CASP+ Study Guide: Industrial Protocols and Modbus Security