Final answer:
Option B: 'Vesta and Ceres are part of the asteroid belt between Mars and Jupiter' is the best fit, as it corresponds with the information provided about NASA's Dawn spacecraft and its mission.
Step-by-step explanation:
Considering the news story provided, the best fit for the empty box would be option B: Vesta and Ceres are part of the asteroid belt between Mars and Jupiter.
This option correctly identifies the two largest main belt asteroids that were visited by NASA's Dawn spacecraft, as mentioned in the supplied text. T
he detailed exploration of these asteroids, which included orbiting both Vesta and Ceres for about a year, has significantly contributed to our understanding of asteroid characteristics and composition, as well as planetary defense.