Final answer:
Europa, known as Jupiter II, is one of the Galilean moons and is about the size of Earth's Moon, famous for its potential subsurface ocean.
Step-by-step explanation:
The moon of Jupiter sometimes known as Jupiter II is Europa. In the hierarchy of Jupiter's moons, it ranks as the second of the Galilean moons when considering their proximity to Jupiter, hence the numerical designation. All Galilean moons were discovered in 1610 by Galileo Galilei, with Europa being one of the four largest along with Io, Ganymede, and Callisto. Europa, along with Io, is about the size of Earth's Moon. Europa is particularly notable for its smooth and cracked icy surface, which suggests the presence of a subsurface ocean, making it a subject of interest in the search for extraterrestrial life within our solar system.