The correct answer is: D. Drain is running with H.F.L.
Uplift pressure is the pressure exerted by a fluid on a surface that is perpendicular to the direction of flow. In an inverted siphon, the uplift pressure is exerted on the roof of the siphon. The uplift pressure is maximum when the drain is running with H.F.L., or high flow level. This is because when the drain is running with H.F.L., the water level in the drain is higher than the water level in the canal. This creates a pressure difference between the two water levels, which results in a higher uplift pressure on the roof of the siphon.
The other options are incorrect because they do not result in a high pressure difference between the water levels in the drain and the canal. When the drain is running dry, there is no water in the drain, so there is no pressure difference. When the canal is running dry, the water level in the canal is lower than the water level in the drain, so there is a negative pressure difference. This negative pressure difference would actually tend to pull the roof of the siphon down, rather than pushing it up.
In conclusion, the uplift pressure on the roof of an inverted syphon is maximum when the drain is running with H.F.L.