What geological layer is known for being strong and approximately 100km thick?

The lithosphere is the correct answer because it refers to the rigid outer layer of the Earth, which includes the crust and the uppermost part of the mantle. This layer is characterized by its strength and is typically about 100 kilometers thick, although its precise thickness can vary based on location, such as being thinner beneath oceanic regions compared to continental areas.

The lithosphere is significant in plate tectonics, as it is divided into tectonic plates that float on the more ductile asthenosphere beneath it. The lithosphere’s rigid properties are crucial for various geological processes, including earthquakes and volcanic activity.

In contrast, the asthenosphere, which lies directly beneath the lithosphere, is a semi-fluid layer that allows the lithospheric plates to move but is not characterized by the same level of strength. The outer core and inner core, while they are critical components of the Earth's layered structure, are not relevant to the question of a strong layer approximately 100 kilometers thick. The outer core is liquid and primarily composed of iron and nickel, while the inner core is solid and extremely dense, with both layers being much deeper than the lithosphere.