Explain at which point in the diagram the earth is at winter solstice, summer solstice, spring equinox, and fall equinox. give the date for each.

On the summer solstice, the sun will shine directly on the Tropic of Cancer (23.5 degrees north ) , so the summer solstice should be picture B. While on the winter solstice, the sun will shine directly on the Tropic of Capricorn (23.5 degrees south ), so the winter solstice is picture D.
Because earth revolves in counter clockwise direction around the sun, so the spring equinox will be picture A and the fall equinox will be picture C.
As for the dates, the spring equinox is on March 20 or 21, the summer solstice is on June 22 or 23, the fall equinox is on September 23 or 24, the winter solstice is on December 22 or 23.