The effective width of a column strip in a flat slab is half the width of the panel. This means that the load from the column is distributed over a width equal to half the width of the slab panel.

Explanation:
1. Flat slabs: Flat slabs are a type of concrete floor system that does not have beams between columns. The load from the columns is directly transferred to the slab, which distributes the load to the supporting columns.

2. Column strip: In a flat slab system, the floor is divided into strips called column strips and middle strips. The column strips are the areas around the columns where the load from the columns is concentrated.

3. Load distribution: The load from the columns needs to be distributed effectively to avoid excessive stress concentrations. This is achieved by considering the effective width of the column strip.

4. Effective width: The effective width of the column strip is defined as the distance from the centerline of the column to the edge of the strip where the load is distributed. In the case of a flat slab, the effective width of the column strip is considered as half the width of the slab panel.

5. Why half the width? When the load from the column is distributed over a wider area, the stress concentrations are reduced, resulting in a more efficient load transfer. By considering half the width of the slab panel as the effective width, the load from the column is distributed over a larger area, reducing the stress on the slab.

6. Other options: The radius of the column (option c) and the diameter of the column (option d) are not directly related to the effective width of the column strip. These are dimensions of the column itself and do not determine the width over which the load is distributed. Option b suggests that the effective width is one-fourth the width of the panel, which is incorrect.

In conclusion, the effective width of a column strip in a flat slab is half the width of the panel. This ensures a more efficient load distribution and reduces stress concentrations in the slab.