Answer: B
Correct answer: A hygrometer
A hygrometer measures the water vapour content of the air. The classic form is the wet-and-dry-bulb pair, in which one thermometer is kept damp by a wick: evaporation cools it, and the gap between the two readings gives the relative humidity from a table. The drier the air, the faster the evaporation and the wider the gap. Note the near-identical distractor. A hydrometer, spelt with a d, measures the density of a liquid by how far it floats in it - it has nothing to do with the atmosphere, and is used to test battery acid or the sugar content of a brewing wort. A barometer measures atmospheric pressure, and an anemometer measures wind speed.