D)    a depression

This is a common topographic symbol for a depression:

Refer to the map below for other features on the map example used in Nature IQ, explanations are below the map.

Drawn Contour Lines

• Steep slopes - contours are closely spaced
• Gentle slopes - contours are less closely spaced
• Valleys - contours form a V-shape pointing up the hill - these V's are always an indication of a drainage path which could also be a stream or river.
• Ridges - contours form a V-shape pointing down the hill
• Summits - contours forming circles
• Depressions - are indicated by circular contour with lines radiating to the center

Source: http://www.princeton.edu/~oa/manual/mapcompass.shtml