CH₃CH₂COO– Na+ $\stackrel{NaOH,+?}{\underset{Heat}{⟶<!--\; ⟶\; -->}}$CH₃CH₃ + Na₂CO₃.

Consider the above reaction and identify the missing reagent/chemical.

The correct option is CaO.

Explanation:
The given chemical equation represents the decarboxylation of sodium propanoate to form ethane:

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{COO}}^{-}{\mathrm{Na}}^{+}+\mathrm{NaOH}\stackrel{\mathrm{CaO},\text{Heat}}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_3+{\mathrm{Na}}_2{\mathrm{CO}}_3$

This reaction is known as soda-lime decarboxylation, where a sodium salt of a carboxylic acid is heated with soda lime. Soda lime is a mixture of sodium hydroxide ($\mathrm{NaOH}$) and calcium oxide ($\mathrm{CaO}$) in a 3:1 ratio.
In this process, carbon dioxide is removed from the carboxylic acid salt in the form of sodium carbonate (${\mathrm{Na}}_2{\mathrm{CO}}_3$). Calcium oxide ($\mathrm{CaO}$) keeps the reaction mixture dry and porous, facilitating the reaction.
Thus, the missing reagent/chemical is CaO.