Explanation: A fully charged lead-acid battery cell normally has an electrolyte specific gravity in the range of 1.265 to 1.280. This value reflects the concentration of sulfuric acid in the electrolyte; as the battery discharges, the acid combines with the plates, lowering the specific gravity. When fully charged, the acid concentration is at its peak. The device used to measure the specific gravity of the electrolyte is a **hydrometer**. A hydrometer works by drawing a sample of the electrolyte into a glass tube, where a calibrated float indicates the density (specific gravity) of the liquid. Therefore, option D correctly identifies both the typical specific gravity of a fully charged lead-acid battery (1.280) and the correct measuring device (Hydrometer). Options A and B are incorrect because their specific gravity values are either too high or too low for a fully charged battery, and a voltmeter measures electrical potential, not specific gravity. Option C lists a specific gravity (1.210) that indicates a partially discharged battery, not a fully charged one.