What Is the Reaction for the Standard Enthalpy of Formation

What is the reaction for the standard enthalpy of formation for HCl(g) ?

A) H⁺(aq) + Cl-(aq) → HCl(g)
B) 1/2 H₂(g) + 1/2Cl₂(l) → HCl(s)
C) NH₃(g) + Cl₂(g) → NH₂Cl(g) + HCl(g)
D) 1/2 H₂(g) + 1/2Cl₂(g) → HCl(g)
E) 1/2 H₂(l) + 1/2Cl₂(g) → HCl(l)