The correct answer is option 'D', bedrock. Let's understand why bedrock is considered the hardest layer of the soil.

Soil is made up of different layers, also known as soil horizons, that vary in composition, color, and texture. These layers are formed over long periods of time through the weathering of rocks, minerals, and organic matter. The layers of soil are typically categorized into four main horizons: topsoil, subsoil, parent material, and bedrock.

1. Topsoil:
- The topsoil is the uppermost layer of soil and is the most fertile layer for plant growth.
- It is rich in organic matter, nutrients, and microorganisms, which support the growth of plants and provide a favorable environment for their roots.
- The topsoil is relatively loose and easy to dig or cultivate.

2. Subsoil:
- Below the topsoil, there is a layer called subsoil.
- The subsoil is generally less fertile compared to the topsoil and contains fewer organic materials.
- It mainly consists of weathered rocks, clay, and minerals that have leached down from the topsoil.
- The subsoil is usually denser and more compact than the topsoil.

3. Parent Material:
- The parent material is the layer beneath the subsoil and is composed of partially weathered rocks and minerals.
- It is the source material from which the soil is formed.
- The parent material undergoes weathering and other geological processes over a long period to form the different layers of soil.

4. Bedrock:
- The bedrock is the bottom-most layer of the soil profile.
- It is the solid, unweathered rock that lies beneath the other soil layers.
- Bedrock is extremely hard and compact, composed of solid rock that has not undergone significant weathering or erosion.
- It provides a stable foundation for the soil layers above it.

Why is bedrock considered the hardest layer?
- Bedrock is the hardest layer of soil because it is composed of solid, unweathered rock.
- Unlike the other layers of soil that have undergone weathering and decomposition, bedrock remains solid and resistant to erosion.
- It requires heavy machinery or drilling equipment to break through the bedrock for construction or excavation purposes.

In conclusion, while each layer of soil has its own characteristics, bedrock is considered the hardest layer because it is composed of solid, unweathered rock that provides a stable foundation for the soil layers above it.

Bedrock is the bottom layer of soil Horizon which is made up of big rocks. So, it is the hardest layer of the soil.