What does thermal expansion refer to?

Thermal expansion refers to the phenomenon where a material's volume increases as its temperature rises. This occurs because, when temperature increases, the kinetic energy of the molecules within the material also increases, causing them to move apart and take up more space. Most materials expand when heated, and this principle is fundamental in various applications, such as engineering, construction, and physics.

Understanding thermal expansion is critical for designing systems where temperature variations are expected, as unaccommodated expansion can lead to structural failures or malfunctions. Thus, option B accurately describes the concept of thermal expansion, reflecting the behavior of materials in response to temperature changes.