What is commonly used to monitor boiler pressure?

A manometer is an instrument specifically designed to measure the pressure of gases or fluids in a system, making it ideal for monitoring boiler pressure. In a boiler system, maintaining the proper pressure is crucial for efficient operation and safety. The manometer provides an accurate reading of the pressure, allowing operators to make necessary adjustments to maintain the desired levels.

The other options serve different purposes: a thermometer measures temperature, which is not directly useful for monitoring pressure; a galvanometer measures electric current, not pressure; and a hydrometer measures the specific gravity of liquids. Each instrument has its unique application, reiterating why the manometer is the correct choice for monitoring boiler pressure.