Final answer:
The primary components of a manometer are a U-shaped tube filled with mercury, and it is used alongside a barometer to measure gas pressures relative to atmospheric pressure.
Step-by-step explanation:
The main components of a manometer are a U-shaped tube and mercury or another nonvolatile liquid. A manometer is used in conjunction with a barometer when measuring atmospheric pressure. Unlike barometers, which measure atmospheric pressure, manometers measure the pressure of a contained gas. The correct answer to the student's question is B) Liquid column; barometer.
Both devices use mercury due to its high density and non-volatile nature, which allows for precise measurements of pressure. There are two types of manometers: a closed-end manometer, which measures absolute gas pressure when one arm is sealed with a vacuum, and an open-end manometer, where one arm is open to the atmosphere and measures the difference in pressure relative to the atmospheric pressure.