The correct answer is: A. on the surface at which resultant pressure acts.
The center of pressure is the point on a surface where the total force of the fluid pressure acts. It is located by taking the moments of the fluid forces about any two points and setting them equal to each other. The center of pressure is always located below the centroid of the surface, and its depth below the centroid is equal to the product of the pressure and the area of the surface.
Option B is incorrect because the gravitational force acts at the center of mass of the object, not at the surface. Option C is incorrect because all hydraulic forces do not meet at a single point. Option D is incorrect because the metacenter is a point on a floating object that is used to determine its stability.