Where on earth is the only location that the sun can ever be directly overhead?

the two tropics zones

Explanation:

Between the two tropics zones, which includes the equator, the Sun is directly overhead twice per year. Outside the tropic zones, whether to the south or north, the Sun is never directly overhead. Two other significant lines of latitude are the Arctic Circle (around the North Pole) and the Antarctic Circle (around the South Pole).

the dark variety, as they are not visible on the dark tree barks

explanation: