Option a is the correct answer. Aryabhatta was an ancient Indian astronomer and mathematician who was the first to explain that the rotation of the earth on its own axis accounts for the daily rising and setting of the sun. He wrote a treatise on mathematics and astronomy called the Aryabhatiya, which contains many original and remarkable discoveries. In the Aryabhatiya, Aryabhatta stated that the earth is spherical and rotates on its own axis at a constant speed. He also stated that the sun and the moon are also spherical and revolve around the earth in elliptical orbits. He calculated the value of pi as 3.1416 and the circumference of the earth as 24,835 miles. He also explained the causes of eclipses, solstices, equinoxes, and seasons. He also devised various methods for solving algebraic equations, trigonometric problems, and astronomical calculations. Important Tips Brahmagupta: He was a 7th century CE mathematician and astronomer who wrote two influential works: Brahmasphutasiddhanta and Khandakhadyaka. Varahamihira: He was a 6th century CE astrologer and polymath who wrote several works on astronomy, astrology, mathematics, and natural sciences. His most famous works are Panchasiddhantika, Brhatsamhita, and Brihatjataka.