The correct option is A ΔHf=−ΔH1+ΔH22−32ΔH3
Heat of formation of NCl3 is the heat of the reaction of the reaction in which NCl3 is formed from it's pure components- N2,Cl2.
Therefore the Reaction-
(1/2)N2+(3/2)Cl2→NCl3,δHf(NCl3) is obtained by-
Reaction(i) + Reaction(ii)/2 - (3/2)Reaction(iii)
So,
ΔHf=−ΔH1+ΔH22−32ΔH3