The correct answer is B. at center of pressure.
The center of pressure is the point on a surface where the total force of the fluid acting on the surface can be considered to be acting. It is located at the centroid of the area of the surface that is in contact with the fluid.
The resultant of all normal pressures acts at the center of pressure. This is because the normal pressure at any point on a surface is perpendicular to the surface, and the resultant of all forces acting perpendicular to a surface must pass through the centroid of the area of the surface.
The center of pressure is important in the design of ships and other floating objects. The center of pressure must be below the center of gravity of the object, or the object will overturn.
A. at e.g. of body is incorrect because the resultant of all normal pressures does not act at the edge of the body.
C. vertically upwards is incorrect because the resultant of all normal pressures does not act vertically upwards.
D. at metacentre is incorrect because the metacentre is a point that is used to calculate the stability of a floating object, but it is not the point where the resultant of all normal pressures acts.