Below was a question in the 2012 Australian Chemistry Olympiads:
The enthalpy change of formation ($\Delta_\mathrm{f}H^\circ$) for a species at $\pu{298 K}$ is defined as the enthalpy change that accompanies the formation of one of the following of one mole of a substance from its constituent elements in their standard states. Which of the following species has $\Delta_\mathrm{f}H^\circ = \pu{0 kJ/mol}$?
a. $\ce{H2O(l)}$
b. $\ce{Na(g)}$
c. $\ce{Na(s)}$
d. $\ce{CO2(g)}$
e. $\ce{O3(g)}$
Solutions stated the answer is b, but isn't the standard state of $\ce{Na}$ solid?