Explanation: A single lead-acid battery cell has a nominal voltage of approximately 2.1 volts when fully charged and at rest. This voltage is characteristic of the electrochemical reaction between the lead plates and sulfuric acid electrolyte within the cell. For example, a common "12-volt" car battery or amateur radio power supply battery consists of six such cells connected in series (6 cells * 2.1 V/cell = 12.6 V). Option A, 1.5 volts, is the typical nominal voltage for alkaline or carbon-zinc dry cells (like AA or D batteries), not lead-acid. Options C and D, 2.5 and 2.8 volts, are too high for a nominal resting voltage. While a lead-acid cell might momentarily reach these voltages during certain charging phases, its stable, "normal" voltage is 2.1 volts. Sustained operation at these higher voltages can indicate overcharging and potentially damage the battery.