The correct answer is D. All of the above.
Regime conditions in a channel occur when the discharge, channel geometry, and sediment load are in equilibrium. This means that the channel is neither eroding nor depositing sediment, and the flow is neither too fast nor too slow. Regime conditions are important because they can help to predict how a channel will respond to changes in its environment, such as changes in rainfall or land use.
The discharge of a channel is the volume of water that flows through it per unit time. The channel geometry is the shape of the channel, including its width, depth, and slope. The sediment load of a channel is the amount of sediment that is transported by the flow.
All of these factors are important in determining whether or not a channel is in regime. If the discharge changes, the channel may erode or deposit sediment in order to adjust to the new flow conditions. If the channel geometry changes, the flow may become too fast or too slow, and the channel may erode or deposit sediment in order to adjust to the new geometry. If the sediment load changes, the channel may erode or deposit sediment in order to adjust to the new sediment load.
Regime conditions are often used to design channels for irrigation, flood control, and other purposes. By understanding how a channel will respond to changes in its environment, engineers can design channels that are stable and that will not cause problems.