ANS:A - closed perimeter of regular shape through which water flows

1. Closed perimeter of regular shape through which water flows: An orifice typically has a circular, square, or rectangular shape with a closed perimeter. The shape of the orifice can affect the discharge characteristics.
2. The water level of the liquid on the upstream side is below the top of the orifice: For flow to occur through the orifice, there must be a difference in hydraulic head between the upstream and downstream sides. This difference causes the liquid to flow through the orifice.
3. Partially full flow: An orifice usually allows for the flow of a portion of the liquid contained in the vessel. The amount of flow depends on factors such as the size of the orifice, the liquid level, and the properties of the liquid.
4. Prolonged sides having a length of 2 to 3 diameters of the opening in thick wall: This description appears to be specific to certain types of orifices used in practical applications. Orifices often have prolonged sides or a thick wall to ensure structural integrity and prevent distortion or damage during operation.