What do GFCIs monitor to provide protection against electrical shock?

GFCIs, or Ground Fault Circuit Interrupters, function by monitoring the difference in electrical current flowing through the hot and neutral wires. Specifically, these devices are designed to detect any imbalance in current, indicating that some of the electrical flow is going through an unintended path, such as a person who may be in contact with the ground. If the GFCI senses a difference exceeding a predetermined threshold (usually around 4-6 milliamperes), it quickly interrupts the circuit, cutting off the electrical supply to prevent potential shock or electrocution.

The other choices relate to different aspects of electrical systems but do not accurately describe the primary function of a GFCI. Voltage fluctuations can occur for various reasons but are not what GFCIs are monitoring. Resistance of the grounding system does not provide immediate detection of current leakage, and temperature monitoring of wiring is not within the scope of function for GFCIs. Instead, their primary protective mechanism relies solely on the current imbalance between the hot and neutral conductors.