Final answer:
The densities of Callisto and Ganymede suggest that these Jupiter moons are composed primarily of ice and rock rather than being predominantly rocky like Earth's Moon. Their densities are below that of Earth's Moon, which leads to the conclusion that they contain large amounts of ice. The best inference to make is that they have roughly equal volumes of ice and rock.
Step-by-step explanation:
The densities of Callisto and Ganymede, which are 1.8 grams per cubic centimeter and 1.9 grams per cubic centimeter respectively, suggest that these moons are composed of materials with relatively low density compared to rocky planets. Given that the densities of these moons are lower than the density of Earth's Moon, which is about 3.34 grams per cubic centimeter, one can conclude that they are not made predominantly of rock and iron. Instead, they are composed of substantial amounts of ice, as indicated by the density values being greater than 1 g/cm³ (the density of water) but less than that of rocky objects.
Ganymede is differentiated with a rock and metal core and an ice mantle, indicating a mixture of materials, while Callisto's low density implies less rocky and metallic material and more ice in its composition. Neither of them must necessarily be larger than Earth's Moon based on the density information alone. In conclusion, the correct answer is that these two moons must be made of roughly equal volumes of ice and rock. Therefore, option E is the most accurate.