What device automatically adjusts the primary voltage of a line?

The device that automatically adjusts the primary voltage of a line is a regulator. Regulators are specifically designed to maintain a consistent voltage level within an electrical system, compensating for variations in load or supply voltage. They ensure that the voltage remains within predetermined limits, enhancing the reliability and efficiency of the power delivery system. This is particularly important in electrical networks where fluctuating demand or supply can lead to voltage instability.

In contrast, a transformer is primarily used to increase or decrease voltage levels but does not automatically adjust the voltage based on changing conditions. Capacitors are used to store electrical energy and improve power factor but do not actively control voltage levels. An inverter converts direct current (DC) to alternating current (AC) and does not have a function related to automatic voltage adjustment. Thus, the regulator plays a crucial role in stabilizing voltage, making it the correct choice for this question.