Your textbook considers Pluto a planet. Accordingly, the correct answer is D. It has to be different than *any* other planet. Well, Neptune is much like Uranus. Wikipedia says "Neptune is similar in composition to Uranus," A previous answer pointed out that they are both called ice giants.
Venus is much like Earth, according to the Encyclopedia Britannica, "Much less is known about the interior of Venus than about its surface and atmosphere. Nevertheless, because the planet is much like Earth in overall size and density and because it presumably accreted from similar materials (see solar system: Origin of the solar system), scientists expect that it evolved at least a crudely similar internal state. Therefore, it probably has a core of metal, a mantle of dense rock, and a crust of less-dense rock. The core, like that of Earth, is probably composed primarily of iron and nickel, although Venus’s somewhat lower density may indicate that its core also contains some other, less-dense material such as sulfur. "
Mercury is also similar to the other terrestrial planets. It also formed from the material of the inner Solar system.
This leaves Pluto, which almost certainly is made up of rock and ice. (judging from it's density) Like a comet in fact.
