Relative Humidity

The correct answer is option B: relative humidity. Relative humidity refers to the amount of water vapor present in the air compared to the maximum amount of water vapor the air can hold at a specific temperature. It is expressed as a percentage.

Explanation:
When we talk about the amount of water vapor in the air, it can be measured in various ways. The absolute humidity, specific humidity, and relative humidity are three common terms used to describe the moisture content in the air.

Absolute Humidity:
Absolute humidity refers to the actual amount of water vapor present in a given volume of air. It is usually expressed in grams of water vapor per cubic meter of air. Absolute humidity is an important parameter in meteorology and is used to determine the moisture content of the atmosphere.

Specific Humidity:
Specific humidity is the ratio of the mass of water vapor to the total mass of the air parcel. It is expressed as the number of grams of water vapor per kilogram of air. Specific humidity is a useful measure for studying the water cycle and is often used in atmospheric science.

Relative Humidity:
Relative humidity, on the other hand, is a measure of how much moisture is in the air relative to the maximum amount of moisture the air can hold at a given temperature. It is expressed as a percentage. Relative humidity is a commonly used measure to describe how "humid" or "dry" the air feels.

Relative humidity is influenced by both the actual amount of water vapor in the air and the air temperature. Warm air can hold more moisture than cold air, so at the same absolute humidity, the relative humidity will be lower in warmer conditions.

In summary, the amount of water vapor actually present per unit weight of air is known as absolute humidity, while specific humidity represents the ratio of water vapor to total air mass. However, when we talk about the moisture content relative to the maximum capacity of air at a specific temperature, we use the term relative humidity. So, the correct answer to the question is option B: relative humidity.