For a static fluid, the increase of pressure at any point inside the fluid, in a vertically downward direction, must be equal to the product of the Specific weight of the fluid and depth from the free surface.

Pressure Varying Horizontally (for Static Fluids)

For a fluid resting inside a container, pressure does not depend on the shape of the cross-section of the container. Also, the pressure is the same at all points on any horizontal plane considered in the fluid present in the container. Consider three containers of open to the atmosphere, of different shapes where the free surface of the liquids in them are at the same level as shown in the following figure.

Hydro-static law is given by:-

\(\frac{{dP}}{{dz}} = - \rho g\)

∴ P = -ρgz

∴ P = ρgh

where P = pressure above atmospheric pressure and h = height of the point from the free surface.

Specific Weight is the weight of a substance per unit volume.

Specific weight, w = \(\frac{{\:{\rm{mg}}}}{{\rm{V}}}\:{\rm{or}}\:\rho {\rm{g}}\:\)

where

m = mass

V = Volume

∴ The correct answer is specific weight.