**Core Concept**
The question is testing the concept of measuring air velocity, which is crucial in various medical and environmental fields. Air velocity is the speed at which air moves, and it can be measured using different devices.

**Why the Correct Answer is Right**
An anemometer is a device specifically designed to measure air velocity. It works by using rotating cups or vanes to capture the wind, and the speed of the rotation is directly proportional to the air velocity. There are different types of anemometers, including mechanical, thermal, and sonic anemometers, each with its own unique mechanism for measuring air velocity. In medical settings, anemometers are used to measure ventilation rates in operating rooms, isolation rooms, and other areas where air quality is critical.

**Why Each Wrong Option is Incorrect**
**Option A:** A psychrometer is a device used to measure the humidity of the air, not its velocity. While it can be used in conjunction with an anemometer to measure air velocity and humidity, it is not the primary device for measuring air velocity.

**Option C:** A kata thermometer is a device used to measure the temperature of the air, not its velocity. It is typically used in meteorology to measure temperature gradients and is not related to air velocity measurement.

**Option D:** A hygrameter is a device used to measure the humidity of the air over time, not its velocity. It is typically used in meteorology to measure humidity trends and is not related to air velocity measurement.

**Clinical Pearl / High-Yield Fact**
In medical settings, accurate measurement of air velocity is critical for maintaining proper ventilation rates, which can help prevent the spread of airborne pathogens. It is essential to use the correct device, such as an anemometer, to ensure accurate measurements.

**Correct Answer Line**
✓ Correct Answer: B. Anemometer