The correct option is C -396 kJ
S+O2→SO2 ΔHo=−297kJ
SO2+O−2→2SO−3 ΔHo=−198kJ
First on multiplying equation first by 2:
2S+2O2→2SO2 2×ΔHo=−297×2=594kJ
Now, adding both equation: ( New + 2nd )
2S+3O2→2SO3 ΔreactionHo=−594−198=−792kJ
Now, determining the enthalpy for 1 mol of SO3 to determine Enthalpy of formation of SO3, divide the equation by 2.
2S(s)+32O2(g)→SO3(g) ΔformationHo=−7922=−396kJ