• Crystalline solids usually have sharp and well-defined melting points because they have a regular pattern of constituent particles which repeats itself periodically throughout the crystal i.e. long-range order. Therefore they have same distance between them or they have same kind of adjacent neighbours and this regularity in crystal lattices create equal environments for all the particles. Thus the intermolecular forces among them are evenly equal and same amount of thermal energy will be needed to break each interaction b/w particles (atoms, molecules or ions) at the same time.

• Options (i) a regular arrangement of constituent particles observed over a short distance in the crystal lattice and (iii) same arrangement of constituent particles in different directions are entirely untrue for crystalline solids.

• And option (iv) different arrangement of constituent particles in different directions cannot explain the phenomena of sharp melting points of crystalline solids because this is in the side of contrary.