AP EAMCET Engineering 17 Sep 2020 Shift 2 Paper

Higher the stability of a substance, higher will be its standard molar enthalpy of formation value, |∆fH∘|
Among the halides, F− is smallest and I− is largest in size. So, according to Fajan's rule, NaF is mostly ionic in nature.
NaI<NaBr<NaCl<NaF
⟶ Ionic nature
→ Thermal stability
So, NaF has highest value of |∆fH∘|.