ANS:D - All the above.

1. For maximum velocity of flow, the depth of water in the circular channel must be 0.81 times the diameter of the channel: This statement is not accurate. The maximum velocity of flow in a circular channel does not occur when the depth of water is 0.81 times the diameter of the channel. The conditions for maximum velocity are more complex and involve factors such as hydraulic radius, channel roughness, and flow conditions.
2. For maximum velocity, the hydraulic mean depth must be 0.3 times the diameter of the circular channel: This statement is also not accurate. The hydraulic mean depth (or hydraulic radius) is indeed a critical parameter in fluid mechanics, but there is no fixed value like 0.3 times the diameter for achieving maximum velocity. The hydraulic mean depth is calculated differently for different flow conditions and geometries.
3. For maximum discharge, the depth of flow must be 0.95 times the diameter of the circular channel: This statement is the most accurate among the options provided. In many cases, for a circular channel, achieving a depth of flow around 0.95 times the diameter can indeed lead to maximizing the discharge, especially under specific flow conditions.