Introduction:
In civil engineering, a footing is a structural element that serves as the base or foundation for a building or any other type of structure. The purpose of a footing is to distribute the load from the structure to the underlying soil or rock in a safe and stable manner. The thickness of the footing at the edge is an important design consideration to ensure the stability and strength of the structure.

Importance of Footing Thickness:
The thickness of the footing at the edge plays a crucial role in providing sufficient bearing capacity and preventing excessive settlement or failure. When designing a reinforced concrete (R.C.) footing on soil, the thickness at the edge should not be less than a certain minimum value to ensure the structural integrity of the footing and prevent any potential failure.

Reason for Option B:
Option B states that the thickness at the edge should not be less than 15 cm. This requirement is based on engineering standards and guidelines that have been established to ensure the safety and stability of structures. The minimum thickness of 15 cm provides an adequate amount of concrete to resist the applied loads and distribute them to the underlying soil without excessive settlement or failure.

Factors Influencing Footing Thickness:
The minimum thickness of the edge of an R.C. footing is influenced by various factors, including:
1. Load on the footing: The magnitude and type of load that the footing is expected to bear, such as dead loads, live loads, and environmental loads.
2. Soil properties: The strength, bearing capacity, and settlement characteristics of the underlying soil or rock.
3. Footing dimensions: The size, shape, and configuration of the footing, including its width and length.
4. Material properties: The strength, durability, and structural properties of the concrete and reinforcement used in the footing.

Design Considerations:
When designing the thickness of the footing at the edge, engineers typically consider factors such as:
1. Bearing capacity: The thickness should be sufficient to distribute the load from the structure to the soil without exceeding the allowable bearing capacity of the soil.
2. Settlement: The thickness should prevent excessive settlement of the footing, which can lead to structural damage or instability.
3. Structural integrity: The thickness should provide adequate strength and stability to resist bending, shear, and other forces acting on the footing.
4. Construction limitations: The thickness should be feasible to construct and should consider practical constraints such as formwork, reinforcement placement, and concrete workability.

Conclusion:
In conclusion, when designing an R.C. footing on soil, the thickness at the edge should not be less than 15 cm. This requirement is based on engineering standards and guidelines to ensure the stability, strength, and durability of the footing. It is important to consider factors such as load, soil properties, footing dimensions, and material properties when determining the appropriate thickness for the footing. By adhering to these design considerations, engineers can ensure the safe and reliable performance of the structure.