Johannes Kepler, while studying the orbit of Mars, discovered that the paths of the planets are not circular but elliptical. This led to his formulation of the first law of planetary motion, which states that planets move in elliptical orbits with the Sun at one focus.
Among the options provided, none are directly correct in the context of Kepler's first law. However, if we focus on what was revealed about the nature of planetary motion — "most planets have highly elliptical orbits" is the closest answer since it reflects the essence of Kepler's discoveries that led to his laws.
Therefore, the best answer, although it isn't perfectly phrased, would be:
d) most planets have highly elliptical orbits.