Introduction:
Acidic soil is characterized by a low pH level, typically below 7. This type of soil can be problematic for plant growth as it affects nutrient availability and can lead to nutrient deficiencies. To improve the pH level of acidic soil, a base is commonly used to neutralize the acidity and make it more suitable for plant growth.

Base commonly used to treat acidic soil:
The correct base commonly used to treat acidic soil is Calcium Hydroxide (option D).

Explanation:
Calcium hydroxide, also known as agricultural lime or garden lime, is a common base used to treat acidic soil. It is an alkaline substance and has the ability to neutralize the acidity in the soil by increasing the pH level.

How does Calcium Hydroxide work?
When calcium hydroxide is applied to acidic soil, it reacts with the hydrogen ions (H+) present in the soil solution. This reaction results in the formation of water and calcium ions (Ca2+). The calcium ions then combine with carbonate ions (CO32-) naturally present in the soil to form calcium carbonate (CaCO3). Calcium carbonate is a neutral compound that helps to raise the soil pH and reduce its acidity.

Benefits of using Calcium Hydroxide:
Using calcium hydroxide to treat acidic soil offers several benefits:

1. Neutralizes soil acidity: The primary benefit of using calcium hydroxide is its ability to neutralize soil acidity by raising the pH level. This creates a more favorable environment for plant growth.

2. Increases nutrient availability: Acidic soil tends to lock up essential nutrients, making them less available to plants. By raising the pH level, calcium hydroxide helps to release these nutrients, making them more accessible for plant uptake.

3. Improves soil structure: Acidic soil is often compacted and poorly structured. The application of calcium hydroxide can help improve soil structure by promoting better aggregation and allowing for better water infiltration and root penetration.

4. Enhances microbial activity: Many beneficial soil microorganisms prefer a neutral pH environment. By neutralizing soil acidity, calcium hydroxide can promote the growth and activity of these beneficial microorganisms, improving overall soil health.

Conclusion:
In conclusion, calcium hydroxide (option D) is commonly used as a base to treat acidic soil. Its ability to neutralize acidity, increase nutrient availability, improve soil structure, and enhance microbial activity makes it an effective choice for improving the pH level of acidic soil and creating a more suitable environment for plant growth.