The correct answer is C. Summer solstice.
The summer solstice is the day of the year when the Sun reaches its highest point in the sky at noon. It is the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere. The summer solstice occurs on June 21 or 22 in the Northern Hemisphere and on December 21 or 22 in the Southern Hemisphere.
The vernal equinox is the day of the year when the Sun crosses the equator from south to north. It is the first day of spring in the Northern Hemisphere and the first day of autumn in the Southern Hemisphere. The vernal equinox occurs on March 20 or 21.
The autumnal equinox is the day of the year when the Sun crosses the equator from north to south. It is the first day of autumn in the Northern Hemisphere and the first day of spring in the Southern Hemisphere. The autumnal equinox occurs on September 22 or 23.
The winter solstice is the day of the year when the Sun reaches its lowest point in the sky at noon. It is the shortest day of the year in the Northern Hemisphere and the longest day of the year in the Southern Hemisphere. The winter solstice occurs on December 21 or 22 in the Northern Hemisphere and on June 21 or 22 in the Southern Hemisphere.