**Explanation:**

Evaporation is the process by which a liquid, in this case water, changes from its liquid state to a gaseous state. It occurs when the molecules of the liquid gain enough energy to overcome the attractive forces between them and escape into the air as vapor.

The rate of evaporation is influenced by several factors, including temperature, wind, the area of exposed surface, and the humidity of the surrounding air. Let's discuss each factor in detail and explain why the correct answer is option 'D' - the humidity of the surrounding air is higher.

**a) The temperature is higher:**
- When the temperature is higher, the molecules of the liquid gain more energy, which increases their kinetic energy.
- As a result, the molecules move faster and collide with each other more frequently, increasing the chances of escaping into the air as vapor.
- Therefore, a higher temperature generally leads to a faster rate of evaporation.

**b) The wind is stronger:**
- When the wind is stronger, it creates air movement over the surface of the liquid, causing the molecules to disperse more rapidly.
- This movement helps carry the vapor molecules away from the surface, allowing fresh air to come in contact with the liquid, promoting evaporation.
- Consequently, a stronger wind generally leads to a faster rate of evaporation.

**c) The area of exposed surface is greater:**
- A larger exposed surface area provides more space for the liquid molecules to escape into the air as vapor.
- When the surface area is increased, more liquid molecules are exposed to the air, increasing the chances of evaporation.
- Therefore, a greater exposed surface area generally leads to a faster rate of evaporation.

**d) The humidity of the surrounding air is higher:**
- Humidity refers to the amount of moisture or water vapor present in the air.
- When the humidity of the surrounding air is higher, it means that the air is already saturated with water vapor.
- In such conditions, the air cannot hold much more water vapor, making it more difficult for the liquid molecules to escape into the air.
- The presence of a high humidity level creates a lower concentration gradient between the liquid and the surrounding air, reducing the rate of evaporation.
- Thus, when the humidity of the surrounding air is higher, the rate of evaporation is slower.

Based on these explanations, it is clear that the correct answer is option 'D' - the humidity of the surrounding air is higher.