The coefficient of curvature (Cc) is a measure of the uniformity of particle sizes in a soil sample. It is defined as the ratio of the square of the particle size corresponding to 60% passing to the particle size corresponding to 10% passing.

In the case of a well-graded soil, the particle sizes are distributed over a wide range. This means that the soil contains a good mix of coarse and fine particles. A well-graded soil is desirable in many engineering applications because it allows for better compaction and drainage properties.

Given that the coefficient of curvature for a well-graded soil must be in the gauge, we can conclude that none of the given options (a, b, or c) are correct. The correct answer is option 'D' (None of these).

There is no specific range for the coefficient of curvature of a well-graded soil. The value of Cc can vary depending on the specific characteristics of the soil sample. However, in general, a higher value of Cc indicates a wider range of particle sizes and a better gradation.

To determine the coefficient of curvature, a soil sample is sieved through a series of standard sieves with different mesh sizes. The weights of particles retained on each sieve are measured, and the cumulative percentage passing is calculated. From this data, the particle size corresponding to 10% and 60% passing is determined, and the Cc is calculated using the formula mentioned earlier.

It is important to note that the coefficient of curvature is just one of the parameters used to describe the gradation of a soil sample. Other parameters such as the coefficient of uniformity (Cu) and the gradation curve can also provide valuable information about the particle size distribution.

In conclusion, the coefficient of curvature for a well-graded soil does not fall within the given options (a, b, or c). The correct answer is option 'D' (None of these). The coefficient of curvature can vary depending on the specific characteristics of the soil sample, and there is no specific range for a well-graded soil.