The correct answer is Option 4. Explanation & Key Takeaways: • Based on their crystal structures, solids can be classified into the following categories:- • Crystalline solids - • The solids featuring highly ordered arrangements of their particles (atoms, ions, and molecules) in microscopic structures are called crystalline solids. Hence, statement a is correct. • Crystalline solids have a sharp melting point is due to this regular arrangement of constituent particles over a long distance in the crystal lattice. Hence, statement b is correct. • Crystalline solids are anisotropic in nature i.e.their physical properties show different values when measured along with different directions. Hence, statement c is correct. • Examples of crystalline solids include salt (sodium chloride), diamond, and sodium nitrate. • Amorphous solids - • The solids in which the particles are not arranged in any specific order or the solids that lack the overall order of a crystal lattice are called amorphous solids. • The term ‘amorphous’, when broken down into its Greek roots, can be roughly translated to “without form”. • Many polymers are amorphous solids. Other examples of such solids include glass, gels, and nanostructured materials. • An ideal crystal is defined as an atomic arrangement that has infinite translational symmetry in all three dimensions, whereas such a definite definition is not possible for an ideal amorphous solid.