The correct answer is: D. uniform flow.
Normal depth is the depth of flow in an open channel when the flow is uniform and steady. It is the depth at which the water surface is parallel to the channel bed and the velocity of the water is constant.
Steady flow is flow in which the velocity of the water does not change with time. Unsteady flow is flow in which the velocity of the water changes with time. Laminar flow is flow in which the water particles move in parallel layers. Turbulent flow is flow in which the water particles move in a chaotic manner.
Uniform flow is a special case of steady flow in which the velocity of the water is constant and the water surface is parallel to the channel bed. Uniform flow occurs when the slope of the channel bed is equal to the critical slope. The critical slope is the slope of the channel bed at which the flow is just about to become turbulent.
The normal depth is the depth of flow in an open channel when the flow is uniform and steady. It is the depth at which the water surface is parallel to the channel bed and the velocity of the water is constant.