The correct option is B 212 kJ mol−1
The enthalpy of formations of the given gases are :
△fH(H,g)=109 kJ mol−1
△fH(O,g)=124 kJ mol−1
△fH(OH,g)=21 kJ mol−1
We have to calculate △H for the equation,
OH(g)→H(g)+O(g)
△H=△fH(H,g)+△fH(O,g)−△fH(OH,g)
△H=(109+124−21)kJ mol−1△H=212 kJ mol−1