We must write the synthesis reaction for every molecule we have in the question tab.
NH3: (look for it on the table below left)
N2 (g) + 3H2 (g) ===> 2 NH3 (g) ΔH = -91.8 kJ
Al2O3:
4 Al(s) + 3O2 ==>2 Al2O3 (s) ΔH = -3351 kJ
H2O(l):
2 H2(g) + O2(g) ===>2 H2O (l) ΔH = -571.6 kJ
C2H2:
2C(s) + H2(g) ===> C2H2 (g) ΔH = +227 kJ
As we can see the first 3 reactions are exothermic because of the negative ΔH (enthalpy) and the last one is endothermic because ΔH is positive.
Now we must say which is most exothermic (more negative ΔH). The more negative enthalpy, the more stable the molecule.
C2H2 < NH3 < H2O < Al2O3 (most stable)
Answer:
The most stable compound: Al2O3