What observational properties of Uranus's rings show that small moons must orbit among the rings? 

The rings of Uranus are stable, narrow, and have defined edges; therefore, there should be shepherd satellites to keep the material confined to the ring. 

The rings of Uranus are stable, wide, and evenly distributed; therefore, there can be only small moons among the rings. Large ones would destroy the ring system. 

The rings of Uranus are stable, wide, and evenly distributed; therefore, there should be shepherd satellites to keep the material confined to the ring.

The rings of Uranus are stable and have defined edges; therefore, there can be only small moons among the rings. Large ones would destroy the ring system.