The correct statement amongst the following is:

The term 'standard state' implies that the temperature is $\displaystyle 0$°C.

The standard state of a pure gas is the pure gas at a pressure of $\displaystyle 1$ bar and temperature $\displaystyle 273$ K.

$\displaystyle \Delta_f \mathrm{H}_{298}^\theta$ is zero for $\displaystyle \mathrm{O}(\mathrm{g})$

$\displaystyle \Delta_f \mathrm{H}_{500}^\theta$ is zero for $\displaystyle \mathrm{O}_2(\mathrm{~g})$