The gravitational attraction of a planet on an object is directly related to its mass. The less massive a planet is, the weaker its gravitational pull will be on a nearby object.
From the information provided:
- Venus: 4.87 × 10^24 kg
- Mars: 0.642 × 10^24 kg
- Jupiter: 1,898.0 × 10^24 kg
- Neptune: 102.0 × 10^24 kg
Among these planets, Mars has the smallest mass (0.642 × 10^24 kg). This means that Mars would gravitationally attract the spacecraft the least when it flies within 500,000 km of each planet.
Therefore, the correct response is Mars.