The correct option is
C They are isotropic.
Crystalline solids consist of large number of crystals, each of which has well-defined regular shape such as a cube, octahedron or tetrahedron.
Characteristics of crystalline solids:
1. They have flat surfaces that meet at angles that are characteristic of the substance.
2. The regularity of these surface features reflects the high degree of order among the particles (atoms, molecules, or ions) within the crystal.
3. They are produced by slow cooling under controlled condition of liquid.
4. They shows isomorphism and polymorphism
5. These are anisotropic, which can shows variation of physical properties with directions. This is due to the long range ordered arrangement in crystalline solids. Thus, there will be variation in physical property in different direction.
6. Due to long range ordered arrangement, they show sharp melting point and they can be cut clean along specific crystal planes
Thus, option (c) is not a characteristic of crystalline solids.