ANS:A - minimum

For critical depth of flow of water in open channels, the specific energy (E) must be minimum. The critical depth occurs when the specific energy (E) of the flow is minimized. At critical depth, the specific energy is at its lowest point for the given flow conditions. The specific energy (E) of a flow is the sum of the depth of flow (y) and the velocity head (v^2/2g). Mathematically, it is represented as: E=y^2/2​g At critical depth, the flow velocity and the depth of flow combine to produce the lowest specific energy for the given flow rate and channel geometry. Therefore, for critical depth of flow of water in open channels, the specific energy must be minimum.