Step 1: Identify the function of each instrument.
Thermometer: Measures temperature.
Hygrometer: Measures humidity, the amount of water vapor in the air. A psychrometer is a specific type of hygrometer.
Pyrometer: Measures high temperatures remotely by detecting thermal radiation.
Penetrometer: Measures the hardness or consistency of a material (e.g., soil, asphalt, or food) by measuring the penetration depth of a needle or cone.
Therefore, a hygrometer is the appropriate instrument for measuring humidity.