The answer should be C, a plain.
Contour lines are used to indicate the height of the land in a map. The closer the contour lines are, the steeper the land will be, as the height of the land changes rapidly.
Therefore, if the contour lines are drawn far apart from each other, most likely the height of the land doesn't changes that much, it remains the same for a larger area, which means a plain should be the answer, as its height doesn't change as much as the other hills, cliff, or valley does.