What factor resists an initial decline in system frequency following a disturbance?

Inertia is the correct choice because it refers to the characteristic of rotating machinery in power systems, primarily generators, to maintain their speed when there is a change in load or disturbance. When a disturbance occurs, such as a sudden loss of generation or an unexpected increase in demand, the inertia of the generators helps to resist an immediate drop in system frequency. This is because the rotational mass of the generators has momentum that acts to stabilize the frequency temporarily, allowing for a brief period during which the grid can respond to the disturbance by adjusting generation or load before the frequency begins to decline significantly.

The other options, while related to frequency control and system stability, serve different functions. Automatic Generation Control (AGC) is a system used to maintain the desired frequency and power interchange with other areas by adjusting the output of generators, but it reacts to frequency changes rather than resist them initially. Automatic Voltage Regulators (AVR) control the voltage output of generators to ensure stable voltage levels but do not directly impact the immediate frequency response following a disturbance. Frequency bias relates to the desired rate of change of frequency in response to load changes, but it is also not a physical factor that resists the initial decline in frequency like inertia does.