(A) plane
Consider a situation in which a liquid is kept in a solid container. The liquid surface normally gets curved at the point of contact with the container.
The diagram shows a liquid which gets lifted upwards at the point of contact. If viewed from above it is concave.
A tangent is drawn at the point of contact. It is a tangent to the liquid surface and it makes an angle θ with the wall of the container. The angle is measured within the side of the liquid.
This angle is known as angle of contact. Since the angle is acute, the surface is concave.
If the angle were obtuse the surface would have been convex. This happens in the case of mercury kept in a glass vessel.
If the angle is zero, it means the tangent is along the surface of the solid. Obviously the surface will be plane in such a case. This happens when water is kept in glass vessel.
