The enthalpies of combustion of carbon and carbon monoxide are - 393.5 and - 283 kJmol-1 respectively. The enthalpy of formation of carbon monoxide pe

C + O2    ⇋   CO2 ;Δ H = −393.5 kJmol-1

CO + ½ O2⇋   CO2; Δ H = −283.0 kJmol-1

Subtracting both the equation

C + ½ O2   ⇋   CO

 ΔH= -393+283 = −110.5 kJmol-1

The enthalpy of formation of carbon monoxide per mole is  −110.5 kJmol-1

Hence the correct answer is (A)  −110.5 kJmol-1