ANS:C - contour line

Option C is correct because Boundary of the still lake is a contour line. And cross-section of still lake is a level surface. For Boundary of water - contour.
Cross-section of still water -level line.
Limit of water level - horizontal level. The contour line is the correct answer. Because focus on the question he asking about line. Not asking about surface. If it's asked about surface than answer is level surface. If it's asked about the line then the correct answer is contour. Boundary of any water body is taken as zero contour line (as mean sea level) and we know that free water tends to level itself, so when we talk about the surface of free water body that surface is a levelled surface.