Final answer:
Faults are fractures in the Earth's crust that occur when tectonic plates interact at their boundaries, usually leading to seismic activity. They are a result of the plates pulling apart, sliding alongside each other, subducting, or colliding, and represent cracks in the Earth's crust.
Step-by-step explanation:
Faults are one of the key features resulting from the movement and interaction of the Earth's tectonic plates. When these massive sections of the Earth's crust and upper mantle, known as tectonic plates, interact at their boundaries, they can cause a variety of geologic activities. The plates may pull apart, subduct or burrow under one another, slide alongside each other, or collide or jam together. This interaction is crucial as it shapes the Earth's geology, including the creation of mountains, valleys, and other structural landforms.
Specifically, a fault is a crack or break in the Earth's crust along which there has been notable movement. A fault can occur as a single brittle fracture or as part of a more complex fault zone with multiple fractures. The movement along faults can be horizontal, vertical, or oblique, and is often associated with seismic activity, like earthquakes. This happens because the plates can become locked due to friction, and when the built-up stress is released, it can cause tremors as the plates move.
The Earth's crust, being more brittle compared to the mantle, is where faults principally occur. This is because the brittle rocks of the crust can fracture and shift, unlike the more ductile mantle that tends to flow rather than crack. When we consider the various options provided in this context, the best description of a fault is b) cracks in Earth's crust, as this is where the manifestation of such tectonic plate movements is evident in the form of fractures and slippages.