The solstice is
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the point at which the Sun is closest to Earth
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the point at which the Sun reaches its greatest distance north or south of the equator
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the seasons
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the point at which the Sun reaches its greatest distance east or west of the prime meridian
A solstice occurs when the Sun reaches its maximum declination - its farthest point north or south of the celestial equator. The summer solstice happens when the Sun is directly overhead at the Tropic of Cancer (23.5°N), and the winter solstice when it's over the Tropic of Capricorn (23.5°S). Option A describes perihelion (closest approach), not solstice. Option C is too vague - solstices mark the beginning of seasons, not the seasons themselves. Option D incorrectly references longitude instead of latitude.