The most economical section of a circular channel for maximum discharge is obtained when the hydraulic mean depth is 0.29d. Let's understand the reasoning behind this answer.

1. Introduction:
In fluid mechanics, the concept of hydraulic mean depth is essential in determining the efficiency and discharge capacity of a channel. The hydraulic mean depth is defined as the ratio of the cross-sectional area of flow to the wetted perimeter. It is denoted by the symbol "R" or "RH".

2. Definition of Hydraulic Mean Depth:
The hydraulic mean depth (R) can be calculated using the formula:
R = A / P
Where,
R = Hydraulic mean depth
A = Cross-sectional area of flow
P = Wetted perimeter

3. Maximum Discharge:
To achieve maximum discharge in a channel, it is important to minimize the resistance to flow. This can be achieved by optimizing the hydraulic mean depth.

4. Economic Section:
The term "economical section" refers to the section of the channel that requires the minimum amount of material to construct while maintaining the desired flow capacity. It is desirable to minimize the wetted perimeter to reduce the amount of material needed for construction.

5. Analysis of Given Options:
a) Depth of water = 0.95d:
Increasing the depth of water increases the cross-sectional area of flow, but it also increases the wetted perimeter. Hence, it does not necessarily result in the most economical section.

b) Wetted perimeter = 2.6d:
Minimizing the wetted perimeter is desirable for an economical section. However, it does not guarantee maximum discharge.

c) Hydraulic mean depth = 0.29d:
By optimizing the hydraulic mean depth, we can achieve both maximum discharge and an economical section. This value of 0.29d is a result of empirical observations and calculations.

6. Conclusion:
Based on the analysis, it can be concluded that the most economical section of a circular channel for maximum discharge is obtained when the hydraulic mean depth is 0.29d. This value allows for efficient flow while minimizing the wetted perimeter, resulting in an economical design.