Lattice energy is

(a) the energy required to form one mole of an ionic compound from its ions in their gaseous states
(b) the energy required to break apart one mole of an ionic compound into its gaseous ions
(c) the energy required to remove an electron from an atom or ion in its gaseous state
(d) the energy required to add an electron to an atom or ion in its gaseous state

(B) the energy required to break apart one mole of an ionic compound into its gaseous ions

Explanation:

Lattice energy is the energy that would be obtained from the formation of an ionic compound from its gaseous ions. Therefore, the reverse process that responds the question would be the energy required to break apart an ionic compound and convert its component atoms into gaseous ions.

Lattice energy provides a measure of the stability of an ionic compound. When the ions bind to form the crystalline structure, heat (energy) is released. To break the ionic compound into the ions that form it, it is necessary to provide an energy equal to that released when formed, i.e. the lattice energy.