Which of the following measures steam pressure in a steam system?

A gauge is the correct instrument for measuring steam pressure in a steam system. Pressure gauges are specifically designed to provide a direct reading of the pressure within a system, and they often come with calibrated scales that accommodate different pressure ranges typically encountered in steam systems. By indicating the pressure exerted by the steam, gauges play a crucial role in monitoring system performance, ensuring safety, and maintaining operational efficiency.

The other options serve different functions. A manometer, while it can measure pressure, is more commonly used for lower pressure applications and for comparative measurements against atmospheric pressure rather than for steam systems. A siphon relates to fluid mechanics and involves the movement of liquids rather than measuring pressure. A thermometer measures temperature, not pressure, making it unsuitable for determining steam pressure. Each of these instruments serves specific purposes, which is why the gauge is the appropriate choice for measuring steam pressure.