The species which by definition has zero standard molar enthalpy of formation at 298 K is : (A) Br2(g) (B) Cl2(g) (C) H2O(g) (D) CH4(g)

Cl2(g)

We know that chlorine exists in a gaseous state at room temperature. Hence, standard molar enthalpy of formation of Cl2(g) is zero

Therefore, the correct option is (B)