Answer:
1. Main-sequence star of spectral type A
2. Jupiter
3. A one-solar-mass white dwarf
4. The Moon
5. A two-solar-mass neutron star
6. The event horizon of a two-solar-mass black hole.
Step-by-step explanation:
This astronomical objects are ranked is based on their diameter and they are ranked from the largest diameter to the smallest diameter.
Main-sequence star of spectral type A has the largest diameter.
While the event horizon of a two-solar-mass black hole has the smallest diameter.