Which of the following species has an enthalpy change of formation = 0 kJ/mol? 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. $\ce{O3(g)}$ 


Original

Solutions stated the answer is b, but isn't the standard state of $\ce{Na}$ solid?
The correct answer is indeed c. $\ce{Na(s)}$

Since for the heat of formation to be zero, the molecule should be the must abundant chemical formula of the element.

$\ce{Na}$ is a metal and is predominantly found in solid state (I dont think it exists as gas anywhere in the atmosphere of earth).

$\Delta_\mathrm{f}H^\circ$ is a quantity which is taken relative to the heat of formation of the most abundant naturally occuring form of an element.