Final answer:
The correct conclusion is that the Sun was directly overhead at the Tropic of Cancer, as this occurs at noon on the summer solstice when the Sun's declination aligns with the latitude of the Tropic of Cancer.
Step-by-step explanation:
When Mary stood directly in her shadow at 12 o'clock (noon) while on the Tropic of Cancer (23.5 degrees N), it can be definitely concluded that C) The Sun is directly overhead at the Tropic of Cancer.
This occurs on the summer solstice, usually on June 20 or 21, when the Sun's declination is 23° N and aligns with the latitude of the Tropic of Cancer, resulting in the Sun being at its zenith at this latitude. At this position, objects do not cast a shadow at noon because the Sun is directly overhead.