Anisotropy refers to the property of a material exhibiting different physical properties in different directions. In other words, the material behaves differently when measured along different axes.

Option 1: Glass - Glass is isotropic, meaning its properties, such as refractive index and density, remain the same regardless of direction. Therefore, it is not anisotropic.

Option 2: Rubber - Rubber is typically isotropic as well, as its properties tend to be uniform in all directions. It does not exhibit anisotropy.

Option 3: Plastic - Similar to rubber, plastic is often isotropic, with its properties being the same in all directions. Therefore, it is not anisotropic.

Option 4: Quartz - Quartz is an excellent example of an anisotropic material. Its properties, such as refractive index and electrical conductivity, vary depending on the orientation of the crystal structure. This anisotropic behavior is due to the ordered arrangement of atoms within the crystal lattice. Hence, quartz is an anisotropic material.

In summary, only option 4, quartz, demonstrates anisotropic properties, making it the correct answer.