ANS:E - all the above.

1. Hydraulic Gradient: The hydraulic gradient is the slope of the water table or piezometric surface in a confined aquifer. It represents the change in total head per unit of distance in the direction of flow. It's not directly related to the line joining points in piezometer tubes.
2. Piezometric Line: This is the line joining the points to which the liquid rises in vertical piezometer tubes fitted at different cross-sections of a conduit. It represents the pressure head of the fluid at various points along the conduit. It's a fundamental concept in hydrodynamics and is used to analyze the pressure distribution in fluid flow systems.
3. Pressure Grade Line: This term is commonly used in fluid mechanics to denote the line that represents the variation of pressure head along a pipeline or conduit due to friction losses. It's not directly related to piezometer tubes.
4. Hydraulic Grade Line: The hydraulic grade line is a line representing the total head available to the fluid in a conduit, pipe, or open channel. It includes both the pressure head and the elevation head of the fluid. It's related to the concept of energy grade line and is used to determine the energy status of the fluid at different points along the flow path.