Explanation
- A
does not set below the horizon at the Arctic Circle
(a) Does not set below the horizon at the Arctic Circle. On 21 June, the June solstice, the North Pole is tilted toward the Sun; at the Arctic Circle the Sun stays on or above the horizon (midnight sun). That is the official key.
- B
does not set below the horizon at Antarctic Circle
(b) Does not set at the Antarctic Circle. 21 June is polar night beginning in the Antarctic, not midnight sun; the Sun does not stay up there, so this is not the key.
- C
shines vertically overhead at noon on the Equator
(c) Shines vertically overhead at noon on the Equator. Overhead sun at the Equator is equinox (23 March / 23 September), not 21 June, so this is not the key.
- D
shines vertically overhead at the Tropic of Capricorn
(d) Vertically overhead at the Tropic of Capricorn. That is the December solstice. On 21 June the overhead ray is at the Tropic of Cancer, so Capricorn is not the key.
Summary. Official key is (a). 21 June is the Northern Hemisphere summer solstice: overhead sun at 23½° N (Tropic of Cancer), midnight sun from the Arctic Circle poleward, and the opposite season in the Antarctic. Equator overhead and Capricorn overhead belong to other dates. Honour the stored letter (a).