The enthalpy of formation, $∆{\mathrm{H}}_{\mathrm{f}}^{°}$, equals zero at $25°\mathrm{C}$ for which of the following in their standard states?

In their normal states, the enthalpies of the elements are taken to be zero. All elements have zero enthalpies of formation in their normal state ( elements like oxygen, nitrogen in the form of gases, solid carbon in form of graphite etc).