Having constructed the Born-Haber cycles, and having worked out the relevant data, look at how the two cycles differ. There should be one key point that changed enormously, to make the formation of NaCl$_2$ so unfavourable. Think what the change would be for a NaCl$_3$, and whether this would be offset by the lattice enthalpy. Generally, we can draw a conclusion for NaCl$_n$...

Although the Born-Lande equation will be new to almost everyone, this question involves little more than plugging in given data to the equation. Note that the Born-Lande calculates lattice enthalpies based on a purely electrostatic model, and consequently always underestimates the true lattice enthalpy. What extra factor hasn't been taken into account? Why would this factor increase for larger halides?