The correct option is B Both assertion and reason are true but reason is not the correct explanation of the assertion
Hess's law states that the enthalpy of a reaction is the same, whether it takes place in a single step or in more than one step.
According to the Born Haber cycle, the enthalpy of formation is equal to summation of sublimation energy of the metal atom, ionization energy required for the formation of metal cation, bond dissociation enthalpy, electron affinity of non-metal and lattice energy.
So Born Haber cycle is based on Hess's law.
Since it is impossible to determine lattice enthalpies directly by experiment we use Born Haber cycle.