Explanation

Iapetus, one of Saturn’s moons, has a unique surface with a bright and dark hemisphere. This causes its brightness to vary depending on its position relative to Saturn. Iapetus appears brightest when it is west of Saturn, because its brighter trailing hemisphere is facing Earth. When east of Saturn, the darker leading hemisphere faces us, making it dimmer. This distinctive brightness variation is specific to Iapetus and makes it easily identifiable during favorable viewing. So, Iapetus is the moon that appears brightest when west of Saturn.