Young’s modulus and shear modulus are both measures of material elasticity. They describe how a material deforms under stress:
- Young's modulus measures the ability of a material to deform elastically (i.e., stretch or compress) when a tensile or compressive force is applied. This is primarily applicable to solids.
- Shear modulus measures how a material deforms under shear stress (parallel to the surface), and like Young’s modulus, it is also applicable to solids.
Both moduli are applicable to solids, and not gases or liquids, because gases and liquids cannot resist deformation in the same way.
Thus, solids are the correct answer.