Which of the following does a voltage relay monitor?

A voltage relay specifically monitors voltage levels within a system. Its primary function is to ensure that the voltage remains within predefined limits. If the voltage rises above or falls below these thresholds, the relay activates to take corrective action or send alerts, which is crucial for protecting electrical equipment from damage due to overvoltage or undervoltage conditions.

Monitoring voltage levels is vital in maintaining the stability and reliability of electrical systems. Proper voltage regulation helps to optimize the performance of devices and prevents potential failures that could lead to costly repairs or downtime.

While current fluctuations, power factor, and harmonics are important parameters in electrical systems, they are not the focus of a voltage relay's operation. Hence, the correct answer is related directly to the primary function of the voltage relay, which is to monitor and regulate voltage levels.