The correct answer is option 'D', which is the equilibrium moisture content. Let's understand why this is the correct answer.

Equilibrium Moisture Content:
The equilibrium moisture content (EMC) is the moisture content that a wet solid eventually attains when it is dried over a long period of time by unsaturated air of nonzero constant relative humidity. It represents the balance between the moisture content in the solid and the moisture content in the surrounding air.

Explanation:
When a wet solid is exposed to unsaturated air, the moisture from the solid starts to evaporate into the air. Initially, the solid has a higher moisture content compared to the air, so there is a concentration gradient that drives the moisture transfer from the solid to the air. As the drying process continues, the moisture content in the solid gradually decreases.

However, the rate of moisture transfer decreases over time as the moisture content in the solid approaches the equilibrium moisture content. At equilibrium, the moisture content in the solid and the moisture content in the air reach a balance, and there is no further net transfer of moisture between them.

This equilibrium moisture content is influenced by several factors such as temperature, relative humidity, and the nature of the solid material. Different materials have different equilibrium moisture content values at the same relative humidity and temperature conditions.

Significance:
The equilibrium moisture content is an important parameter in various industries such as food processing, pharmaceuticals, and construction. It helps in determining the stability, quality, and performance of the product. For example, in food processing, the equilibrium moisture content of a food product affects its shelf life, texture, and taste.

Conclusion:
In conclusion, the equilibrium moisture content is the moisture content that a wet solid eventually attains when it is dried over a long period of time by unsaturated air of nonzero constant relative humidity. It represents the balance between the moisture content in the solid and the moisture content in the surrounding air. Understanding the equilibrium moisture content is crucial in various industries to ensure product quality and stability.