Step 1: Classify the exact relationship in the given pair: "Thermometer : Temperature" is an Instrument : Measured-quantity relationship, where the second word is the specific physical quantity the instrument is built to measure.
Step 2: Apply this same template to "Barometer : ____" - a barometer is the instrument specifically designed to measure atmospheric pressure.
Step 3: Test each option against the template - "Humidity" is measured by a hygrometer, not a barometer, so it is rejected. "Altitude" is the quantity read by an altimeter, a different instrument, so it is rejected. "Wind" is measured by an anemometer, so it is rejected. "Pressure" is exactly the quantity a barometer measures, matching the Instrument : Measured-quantity template precisely.
Correct option: (4) Pressure.