ANS:D - all the above.

The most economical section of a circular channel for maximum velocity occurs when the hydraulic mean depth is equal to half of the diameter of the circular channel. Among the options provided:

• Depth of water = 0.810 diameter: This condition isn't typically related to the most economical section of a circular channel for maximum velocity.
• Hydraulic mean depth = 0.304 diameter: This is not the condition for the most economical section for maximum velocity. The hydraulic mean depth is related to the average depth of flow, but it's not necessarily related to maximizing velocity.
• Wetted perimeter = 2.245 diameters: While the wetted perimeter is a relevant parameter, it's not directly related to maximizing velocity in a circular channel.

Therefore, none of the options provided seem to fulfill the condition for the most economical section of a circular channel for maximum velocity. The most economical section for maximum velocity occurs when the hydraulic mean depth is equal to half the diameter of the circular channel.