To determine which planet's diameter would be approximately the distance across the United States, it's helpful to know the approximate diameters of the planets:
- Mercury: about 4,880 km
- Jupiter: about 139,820 km
- Uranus: about 50,724 km
- Neptune: about 49,244 km
The distance across the contiguous United States is approximately 4,800 km (about 3,000 miles). Based on these measurements, Mercury's diameter (approximately 4,880 km) is the closest to this distance.
So the correct response is Mercury.