Some crystalline solids are anisotropic because despite showing periodicity they are not exactly the same in all directions. It all depends on the symmetry of the unit cell of the crystal. If its size in the x, y and z direction is the same, it would be isotropic and anisotropic otherwise.

Explanation:
Crystalline solids are classified into four main types, network, molecular, ionic, and metallic, based on the type of bonding that exists between the constituent particles. The correct explanation to help someone understand why chemists might have these groups is provided by option D.

- Lattice structure: All crystalline solids share a lattice structure. A lattice structure is a regular arrangement of particles that repeat in three dimensions. The type of lattice structure that a crystalline solid has depends on the type of bonding that exists between the constituent particles.
- Bonding: The bonds that hold the constituent particles together at the atomic level differ between the four main types of crystalline solids. These differences in bonding affect the physical properties of the solid, such as its density, conductivity, and melting point.
- Elements: The elements that make up the constituent particles also differ between the four main types of crystalline solids. For example, network solids are made up of non-metallic elements, while ionic solids are made up of metallic and non-metallic elements.
- Physical properties: The differences in bonding and elements affect how a solid conducts heat and electricity and its density. For example, network solids have very high melting points and are generally not conductive, while metallic solids are good conductors of heat and electricity.

In summary, the classification of crystalline solids into four main types is based on the type of bonding and elements that make up the constituent particles, which affects the physical properties of the solid.