What physical change occurs in rocks as a response to external forces?

Strain is the correct answer because it refers to the deformation that occurs in rocks as a result of external forces, such as stress from tectonic activity, pressure, or changes in temperature. When rocks experience strain, their shape or volume changes without altering their chemical composition. This deformation can manifest as bending, folding, or fracturing, depending on the intensity of the external forces and the properties of the rocks involved.

Weathering, on the other hand, involves the breakdown of rocks through physical, chemical, or biological processes, but it does not specifically describe the response of rocks to external forces in terms of deformation. Metamorphosis typically refers to a significant transformation in the nature of rocks due to changes in temperature and pressure, leading to metamorphism, which is more than just a physical change. Fusion usually implies melting and the processes involved with magma formation, which also does not pertain to the physical response of rocks to external forces.