Final answer:
Lattice energy is the energy released when a crystal lattice forms and is the key factor in determining the stability of an ionic compound, with higher lattice energies leading to higher melting points and hardness.
Step-by-step explanation:
Lattice energy is the energy released when a crystal lattice forms, according to option b). This energy is significant because it is the most important factor in determining the stability of an ionic compound. Lattice energy is the measure of the strength of the electrostatic attractions between oppositely charged ions in a crystal structure. The higher the lattice energy, typically the higher the melting point and hardness due to the need for more thermal energy to overcome the forces that hold the ions together. Lattice energies cannot be directly measured but can be calculated using the Born-Haber cycle and are related to the ionic charges, the ions' arrangement in the crystal, and the internuclear distance.