The speed of a planet in its orbit around the Sun generally decreases with increasing distance from the Sun. This is described by Kepler's laws of planetary motion and Newton's law of gravitation. Among the given options, Jupiter is indeed the largest planet, Mercury is the smallest, and Venus is the brightest (due to its thick atmosphere reflecting sunlight). However, Mars is not the slowest moving planet. The slowest orbital speeds are for the dwarf planet Pluto (though often not considered a major planet anymore) and the farthest major planet, Neptune. Mars is one of the inner planets and moves faster than the outer gas giants.