Relative humidity can be measured with a/an

Step 1: Analyze the question The question asks which instrument measures relative humidity, which is the amount of water vapour in the air compared with the maximum the air can hold at that temperature.

Step 2: Explain the concept Many instruments in ecology and physics have similar-sounding names, so the prefix of each word helps:

• Anemometer – "anemo" means wind. It measures wind speed.
• Barometer – "baro" means pressure. It measures atmospheric pressure.
• Hydrometer – "hydro" means water (liquid). It measures the density or specific gravity of liquids, such as in a car battery or in milk.
• Hygrometer – "hygro" means moisture or humidity. It measures the moisture content of the air.
• Photometer – "photo" means light. It measures light intensity.

Step 3: Solve Relative humidity is a measure of moisture in the air, so we need the instrument whose name comes from "hygro". That is the hygrometer. Common types include the wet-and-dry bulb hygrometer and the hair hygrometer. In the wet-and-dry bulb type, the difference between the two thermometer readings is used with a table to find the relative humidity.

Step 4: Answer The correct option is D. Hygrometer.

Tip: Match the Greek root to the thing being measured:

• Hygro = humidity
• Baro = pressure
• Anemo = wind
• Photo = light
• Hydro = liquid density

Hydrometer and hygrometer are the pair most often confused in exams. Remember "Hy-Gro = Gas (air) moisture", while "Hydro = liquid".