In Terms of the Enthalpy of Formation, Which of the Following

In terms of the enthalpy of formation, which of the following compounds is the most unstable compared to its elements under standard conditions?

A) KCl (s) , $\Delta$ H_f ^$\degree$ = - 435.9 kJ / mol
B) PH₃ (g) , $\Delta$ H_f ^$\degree$ =+ 5.4 kJ/mol
C) Fe₃O₄ (s) , $\Delta$ H_f ^$\degree$ =- 1117.1 kJ/mol
D) NiO(s) , $\Delta$ H_f ^$\degree$ = - 239.7 kJ/mol
E) ICl(g) , $\Delta$ H_f ^$\degree$ = +17.8 kJ/mol