Final answer:
Without the period of revolution or the mass of the star, the velocity of a planet based solely on its semi-major axis cannot be accurately determined. Earth's average orbital speed serves as a general reference point.
Step-by-step explanation:
The question asks us to determine the velocity of a planet given its semi-major axis. A precise velocity cannot be calculated without additional information, such as the planet's period of revolution or the mass of the star it orbits. However, we can estimate the orbital speed using Kepler's laws, knowing that for planets in our solar system, velocities are on the order of a few to tens of kilometers per second. For instance, Earth's average orbital speed is about 29.78 km/s, with a semi-major axis of approximately 150 million km. If the semi-major axis provided is of a planet in our Solar System, then the value could be in a similar range, thus options (c) and (d) could be reasonable estimates, although not accurate without further data.