Crystals in general, may have grater refractive index or electrical conductivity in one direction than another. it is called:

Explanation

.A is incorrect since polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. Polarity has no link to the refractive index or electrical conductivity in one direction. B is incorrect since optical activity is the ability of a substance to rotate the plane of polarization of a beam of light that is passed through it. Optical activity has no link to the refractive index or electrical conductivity in one direction. C is correct since the physical properties that depend upon the direction are called anisotropic properties and the phenomenon is referred to as anisotropy. The physical properties of crystalline solids like refractive index, coefficient of thermal expansion, and electrical and thermal conductivities are sometimes anisotropic for some crystals. The variation in these properties with direction is because the orderly arrangement of the particles in crystalline solids is different in different directions. For example, the electrical conductivity of graphite is greater in one direction than in another. D is incorrect since the plane surfaces formed by the breaking of a crystal are known as crystal cleavage planes. The angle between the adjacent cleavages of a crystal of a substance are the same but are different in the crystals of different substances. In other words, these angles are characteristics of crystalline solids. Cleavage itself is an anisotropic property but not what the question is asking for.