Which force causes molecules to change due to applied pressure?

The correct choice is stress, as stress is defined as the internal force per unit area that arises within materials when they are subjected to external loads or pressures. When pressure is applied to a material, it can cause deformation, which can alter the shape or size of the molecules within that material. This change occurs as the molecular structure responds to the external forces, leading to a realignment or compression of the particles that makes up the material.

In the context of mechanics, stress is fundamental in analyzing how materials behave under load, helping to predict failure or deformation in structures. When stress exceeds a certain threshold, it can lead to significant changes in the material's properties or integrity.

Understanding stress is crucial for professionals in fields related to engineering, materials science, and assistive technology, as it allows them to assess how devices and structures will perform under various conditions, ensuring safety and functionality.