Concept:A barometer is the standard instrument used to measure air pressure.
Explanation:To compare air pressure at different places, you need to take pressure readings at each location.
A barometer gives the atmospheric pressure reading directly.
A hygrometer measures humidity, not air pressure.
A photometer measures light intensity.
An air pressure disc is not a standard or appropriate instrument for this purpose.
Therefore, the only suitable instrument for comparing air pressure is a barometer.
Answer:B. Barometer