A sudden motion of Earth's crust caused by faulting or volcanic activity.

• A. Curie Point
• B. Echo Sounder
• C. Density
• D. Earthquake

Answer: (D)

The main idea being tested is identifying a seismic event as a sudden motion of the Earth's crust due to stress release. An earthquake is the rapid release of energy that occurs when rocks fracture and slip along faults or during volcanic activity, sending seismic waves through the Earth and causing ground shaking. This definition is why it’s the best answer: it specifically describes the abrupt crustal movement caused by faulting or volcanic processes.

The other terms describe different concepts: Curie Point is about the temperature at which magnetic properties of rocks change, not about movement of the crust; an echo sounder uses sound waves to measure depth, not to describe tectonic events; density is simply a material property (mass per volume) and does not refer to any sudden crustal motion.