In terms of thickness, how deep can the asthenosphere be estimated below the lithosphere?

The correct answer reflects the range of the asthenosphere's depth beneath the lithosphere effectively. The asthenosphere is a semi-fluid layer of the Earth located beneath the lithosphere, which includes the crust and the uppermost portion of the mantle. Typically, its thickness can vary significantly depending on the region.

Estimates place the asthenosphere's depth anywhere from approximately 50 miles to several hundred miles below the surface, particularly in regions where tectonic activity is prominent. This variability is largely influenced by geological processes such as subduction and volcanism, which can alter the thickness of the asthenosphere.

While the other options suggest narrower or lesser ranges, they do not encompass the full scope of the asthenosphere’s depth. The selection indicating a depth ranging from 50 to several hundred miles accurately acknowledges the complexities encountered in different geological settings and variations across the Earth’s structure.