An open loop control system requires which of the following?

In an open loop control system, the system operates without feedback, meaning it does not monitor the output and adjust the input based on that output. Because of this, constant operator attention becomes critical. An operator must continuously monitor the system to ensure that it is functioning correctly and to make any necessary manual adjustments, as there is no automatic feedback mechanism to correct any deviations from desired performance.

The requirement for constant operator attention is necessary because open loop systems are more prone to errors or changes that could affect the system's performance. The operator must be vigilant to ensure that conditions remain as expected or to intervene when necessary.

In contrast, options that involve sophisticated sensors or feedback mechanisms are not requirements for an open loop system, as it fundamentally lacks these elements. Thus, it is essential for the operator to actively manage and oversee the process to maintain desired outcomes.