The correct answer is option C: 50 to 60%.

Brick earth is the main material used in the production of bricks. It is a type of soil that is suitable for making bricks due to its composition and properties. One of the key components of brick earth is silica, which is a major determinant of the quality of the bricks produced.

Silica is a mineral that is commonly found in soil, rocks, and sand. It is a key ingredient in the formation of clay minerals, which are the primary building blocks of bricks. The percentage of silica in brick earth can vary depending on the source and composition of the soil.

Here is a breakdown of the percentage of silica in good brick earth:

1. High Silica Content:
- Silica content above 70% is generally considered too high for brick production. Such high silica content can lead to excessive shrinkage and cracking during the drying and firing process. Bricks with high silica content are also more prone to thermal expansion and contraction, which can cause structural issues in the long run.

2. Low Silica Content:
- Silica content below 50% is generally considered too low for brick production. Bricks with low silica content may lack the necessary strength and durability required for construction purposes. They may also be more susceptible to moisture absorption and weathering.

3. Optimum Silica Content:
- The ideal range for silica content in good brick earth is between 50% to 60%. This range provides a balance between strength, durability, and workability. Bricks made from such brick earth will have adequate strength to withstand the load and weathering conditions, while also being easy to shape and fire.

It is important to note that silica is not the only factor that determines the quality of brick earth. Other factors such as clay content, silt content, organic matter, and moisture content also play significant roles in determining the suitability of brick earth for brick production.

In conclusion, a good brick earth should have a silica content between 50% to 60% for optimal brick production. This range provides the right balance of strength, durability, and workability for the bricks.