What does a hygrometer measure?

A hygrometer is an instrument used to measure the humidity of the air, that is, the amount of water vapour present in the atmosphere.

Option A: Heat
Incorrect. Heat is a form of energy and is measured using appropriate temperature or calorimetric methods, not a hygrometer.

Option B: Humidity
Correct. A hygrometer measures humidity, commonly expressed as relative humidity in percentage (%).

Option C: Force
Incorrect. Force is measured using a force meter or spring balance, and its SI unit is the newton (N).

Option D: Radiation
Incorrect. Radiation is measured using instruments such as radiometers or radiation detectors, depending on the type of radiation.

Hence, Option B — Humidity is the correct answer.