The correct answer is: C. the sun returns to equator.
The spring season in the north temperate zone is when the sun returns to the equator. This happens on March 20 or 21 each year, and is known as the vernal equinox. On this day, the sun crosses the equator from south to north, and the length of day and night are equal.
Option A is incorrect because the sun is directly overhead the equator at the summer solstice, which occurs on June 20 or 21 each year.
Option B is incorrect because the sun is directly overhead the Tropic of Cancer at the summer solstice. The Tropic of Cancer is a latitude line that is 23.5 degrees north of the equator.
Option D is incorrect because the sun is at the Tropic of Cancer during the summer solstice. The Tropic of Cancer is a latitude line that is 23.5 degrees north of the equator.