Which layer of the Earth is responsible for tectonic activity such as earthquakes and volcanoes?

The lithosphere is the correct answer because it encompasses the rigid outer layer of the Earth, which includes the crust and the uppermost part of the mantle. This layer is divided into tectonic plates that float on the more fluid asthenosphere beneath. The movement of these tectonic plates is what causes geological activity such as earthquakes and volcanic eruptions.

When these plates interact at their boundaries, they can push against each other, pull apart, or slide past one another, leading to the buildup of stress and ultimately causing earthquakes. Similarly, volcanic activity is often associated with the movement of plates, particularly at divergent boundaries where magma rises to the surface as plates separate or at convergent boundaries where one plate is forced under another, melting rock and creating magma.

In contrast, the outer core and inner core are composed of molten and solid metals, respectively, and while they contribute to the Earth's overall structure and dynamics, they do not play a direct role in tectonic activity. The crust, while being part of the lithosphere, does not account for the dynamic interactions that occur among tectonic plates; thus, it’s the lithosphere as a composite layer that is responsible for such geological phenomena.