ANS:D - all the above.

1. The free water surface of an open channel is not parallel to the bed of the channel: In uniform flow, the water surface in an open channel remains parallel to the bed of the channel. However, in non-uniform flow, the water surface may not be parallel to the bed. This typically occurs when the velocity of the flow changes along the length of the channel, causing variations in the water surface elevation.
2. The head needed to overcome frictional resistance is less than the drop in elevation of the channel bed: In non-uniform flow, if the head required to overcome frictional resistance (i.e., energy loss due to friction along the channel) is less than the drop in elevation of the channel bed, it indicates that the flow is unable to maintain its energy sufficiently to prevent the water surface from dropping. This results in an imbalance between the energy needed to overcome friction and the available energy, leading to non-uniform flow.
3. The head needed to overcome frictional resistance is more than the drop in elevation of the channel bed: Conversely, if the head required to overcome frictional resistance is more than the drop in elevation of the channel bed, it suggests that the flow has more energy available than necessary to overcome frictional losses. In this case, the excess energy may cause the water surface to rise, leading to non-uniform flow conditions.