The correct answer is option 3: Twice a year. When we say that the sun shines vertically on the Equator, it means that the sun is directly overhead at noon, casting no shadows. This occurs during the equinoxes, which are the two times in a year when the length of day and night are approximately equal. The first equinox happens around March 20th, known as the vernal equinox, and the second occurs around September 22nd, known as the autumnal equinox. During these times, the Earth`s axis is neither tilted towards nor away from the sun, resulting in the sun`s rays striking the Equator directly. During other times of the year, the sun is at an angle, causing shadows to be cast. Therefore, the sun shines vertically on the Equator twice a year.