The correct answer is C. 85th percentile.
The 85th percentile speed is the speed at which 85% of the drivers travel at or below. It is used in highway geometric design purposes to ensure that the design speed is safe for the majority of drivers.
The 15th percentile speed is the speed at which 15% of the drivers travel at or below. It is used in highway geometric design purposes to ensure that the design speed is not too high for the minority of drivers who drive slowly.
The 50th percentile speed is the median speed. It is not used in highway geometric design purposes because it does not provide enough information about the range of speeds that drivers travel at.
The 98th percentile speed is the speed at which 98% of the drivers travel at or below. It is not used in highway geometric design purposes because it is too high for the majority of drivers.