Answer: C. Contour line
The boundary of water of a still lake represents a contour line.
Contour line: Counter line is the line joining the points having equal elevation. The boundary of water of a still lake represents equal elevation at all the points, So the boundary of water of a still lake represents the contour line.
Level surface: The level surface is the surface that is parallel to the earth's curvature at all points. It is always perpendicular to the plumb line.
Horizontal surface: It is a flat surface that is tangential to the level surface. It is perpendicular to the plumb line at the point at which it is tangential to the level surface.
Note: In the case of plain surveying, the level surface and horizontal surface are the same.
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