ANS:D - All of these.

1. Depth of Flow: The depth of flow refers to the vertical distance between the water surface and the channel bottom. It is one of the key parameters used to describe the state of flow in an open channel. The depth of flow determines the amount of water present in the channel at any given point.
2. Velocity of Flow: The velocity of flow refers to the speed at which the water moves within the channel. It is the rate at which the water molecules travel past a specific point in the channel over a certain period of time. Velocity is usually measured in meters per second (m/s) or feet per second (ft/s) and can vary depending on factors such as channel slope, roughness, and flow rate.
3. Rate of Flow: The rate of flow, also known as discharge or volumetric flow rate, represents the volume of water passing through a cross-section of the channel per unit of time. It is typically measured in cubic meters per second (m³/s) or cubic feet per second (ft³/s). The rate of flow is the product of the velocity of flow and the cross-sectional area of the channel.