What device is used to measure the strength of battery acid?

The hydrometer is the correct device for measuring the strength of battery acid. This tool operates based on the principle of buoyancy and is designed to assess the specific gravity of liquids. In the context of battery acid, the specific gravity measurement can indicate the state of charge or concentration of the sulfuric acid in a lead-acid battery. Higher specific gravity readings often correlate with a fully charged battery, while lower readings suggest a discharged battery.

The other devices listed serve different functions: a manometer measures the pressure of gases or liquids; a volt meter measures electrical potential difference (voltage); and a thermometer measures temperature. Each of these instruments is specialized for its own purpose and would not provide information about the concentration or strength of battery acid. Hence, the hydrometer is distinctly suited for assessing the health and status of battery acid.