Which layer of the Earth is made up of molten rock?

The mantle is the correct choice as it is primarily composed of semi-solid rock that behaves like a viscous fluid over geologic time, allowing for the movement of tectonic plates. Importantly, while the mantle contains solid rock, it also has sections that can be partially molten, especially at greater depths and in specific areas like mantle plumes. This molten aspect is significant as it plays a crucial role in processes such as magma formation, which can lead to volcanic activity.

In contrast, the crust is the outermost layer of the Earth, composed of solid rock and is not molten. The outer core, while providing a significant amount of molten material, is primarily composed of liquid iron and nickel and is distinct from the mantle's rock composition. The inner core, on the other hand, is a solid layer composed primarily of iron, which remains solid due to the immense pressure at that depth. Thus, the mantle is recognized for having areas of partial melt, making it the best answer for this particular question.