A

Step 1: Identify the forces acting on the liquid.
When a container holding a liquid is on an inclined plane, there are two main forces acting on the liquid: gravitational force and the normal force from the wall of the container.

Step 2: Analyze the effect of slope.
As the vessel slips down the incline, the liquid inside will adjust its surface to maintain equilibrium. The free surface of the liquid will have a slope that is equal to the angle of inclination of the incline due to gravity acting perpendicular to the surface of the liquid.

Step 3: Determine the configuration of the surface.
The liquid will take a shape where the surface lies perpendicular to the net direction of the gravitational force acting on it, which is a straight line parallel to the incline. Thus, the surface of the liquid tilts at the same angle as the incline.

Conclusion: Therefore, the surface-configuration of the liquid in a vessel that is slipping down an inclined plane is such that it is parallel to the incline of the plane.