Earth's average distance from the Sun: 1.496times 10^8 km a Kepler's second law b Retrograde motion c Law of universal gravitation d 1 astronomical unit

The correct answer is d) 1 astronomical unit.<br /><br />The Earth's average distance from the Sun is approximately 1 astronomical unit (AU), which is equal to $1.496\times 10^{8}$ km. An astronomical unit is a unit of measurement used to express distances within our solar system. It is defined as the average distance between the Earth and the Sun, which is about 149.6 million kilometers or 93 million miles.<br /><br />Option a) Kepler's second law refers to the law of equal areas, which states that a planet's orbit around the Sun is an ellipse with the Sun at one of the two foci, and the line joining the planet to the Sun sweeps out equal areas during equal intervals of time.<br /><br />Option b) Retrograde motion refers to the apparent backward motion of a planet when observed from Earth. This occurs when the planet is moving in the opposite direction to most other planets in its orbit.<br /><br />Option c) The law of universal gravitation states that every point mass attracts every other point mass by a force acting along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them.<br /><br />Therefore, the correct answer is d) 1 astronomical unit.