The correct answer is C. Industrial Control System (ICS). The study guide states that ''ICS is a main component of OT'' and ''consists of systems used for monitoring and controlling industrial processes.'' It also explains that ICS includes various devices, systems, controls, and networks that manage industrial processes, and that the most common types are SCADA and distributed control systems (DCS). This makes ICS the primary OT component for monitoring and controlling industrial processes.

The other options are related OT components, but they are not the best answer to this wording. SCADA collects real-time data and helps visualize and control the OT environment, but it is described as a system within the broader ICS structure. PLC devices collect and transmit real-time data and connect sensors and RTUs to SCADA, while IIoT refers to sensors, actuators, and other connected field devices. Therefore, the overarching main OT component for monitoring and controlling industrial processes is ICS.