Which layer of Earth's interior is solid and composed mainly of iron and magnesium silicate?

The correct answer is the mantle, which is the layer of Earth's interior that is solid and primarily composed of iron and magnesium silicate minerals. The mantle lies beneath the crust and extends to the outer core, making up the majority of Earth's volume. Its composition is characterized by silicate minerals rich in iron and magnesium, which contributes to its solid state under the high pressures and temperatures found at these depths.

The mantle behaves in a plastic manner, allowing for convection currents to occur, which play a critical role in driving plate tectonics on the Earth’s surface. These characteristics make the mantle a key component in the dynamics of the planet's geology.

The crust, on the other hand, is the outermost layer, primarily composed of lighter silicate minerals such as quartz and feldspar, and does not contain significant amounts of iron and magnesium silicate. The core is largely made up of iron and nickel and consists of a solid inner core and a liquid outer core, so it does not fit the description of being composed solidly of iron and magnesium silicate. The outer core is also liquid and primarily composed of molten iron and nickel, which does not match the criteria in the question.