Br2(g)
Cl2(g)
CH4(g)
H2O(g)
The standard molar enthalpy of formation of an element in its standard state is taken as zero. Since the standard state of Cl2 is gas, so it's ΔfH = 0. Bromine and water exist in the liquid state at 298 K. Methane is not an elemental species. Hence, the correct option is B.