Final answer:
d. Gravel is the roughest material in Manning's Roughest Coefficients.
Step-by-step explanation:
In Manning's Roughness Coefficients, the roughness of a material is represented by a dimensionless value denoted as 'n'. A higher value of 'n' indicates a rougher material.
In the given options, the roughest material among Asphalt (n=0.015), Claytile (n=0.012), Concrete (n=0.011), and Gravel (n=0.030) is d. Gravel because it has the highest 'n' value.