Write the balanced chemical equations for the following reactions.
Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogen carbonate.

Prior to writing a balanced chemical equation, balance the atoms on both sides of the reaction other than oxygen and hydrogen. The oxygen atoms are then balanced. The hydrogen atoms will then be balanced, and the balance of all the atoms on both sides of the process will be verified once more.

Given that sodium carbonate produces sodium chloride and sodium hydrogen carbonate when reacting with hydrochloric acid in an equal molar ratio. The following outcome will occur-
${\mathrm{Na}}_2{\mathrm{CO}}_3+\mathrm{HCl}\to \mathrm{NaCl}+{\mathrm{NaHCO}}_3$
Without including any stoichiometry, we just listed the reactants and products that were involved in the above reaction. Try to balance the equation now by starting with the atoms that aren't oxygen and hydrogen.
As we can see, the reaction is already in balance because there are 2 sodium atoms, 1 carbon atom, and 1 chlorine atom on both sides of the reaction.
Similarly, oxygen and hydrogen have an equal number of atoms on both sides of the reaction, therefore there is no need to balance them in this case.
The equation in this scenario will remain the same because all atoms were already in balance and we haven't changed anything. Thus, the final balanced reaction will be- 
 ${\mathrm{Na}}_2{\mathrm{CO}}_3+\mathrm{HCl}\to \mathrm{NaCl}+{\mathrm{NaHCO}}_3$
Remember to check whether the previously balanced atom is still the same (balanced) or has changed (unbalanced) before moving on to the next step of balancing the atoms, and then proceed so that all of the atoms (which are balanced in the previous steps) are balanced alongside the atom that is balancing at the moment. After completing each stage, check to see if the reaction is balanced overall. This is because while balancing one atom, the balance of another atom may be disturbed.

Hence, the correct answer is option D.