Which of the following protocols is mentioned as being commonly used in control systems?

Modbus TCP is a widely used protocol in industrial control systems, enabling communication between devices such as PLCs and SCADA systems over Ethernet networks. It is an adaptation of the classic Modbus protocol to TCP/IP networks and is explicitly referenced in ISA/IEC 62443 documentation as a common protocol for IACS communications. FTP, HTTP, and SMTP are general IT protocols and not primarily associated with industrial control communications.