Balance the chemical equation for the reaction between magnesium and hydrochloric acid.

Magnesium ($\text{Mg}$) reacts with hydrochloric acid ($\text{HCl}$).
This is a single displacement reaction where a metal reacts with an acid to produce a salt and hydrogen gas.
The salt formed will be magnesium chloride ($\text{MgCl}_2$) because magnesium has a valency of 2+ and chlorine has a valency of 1-.
The gas produced is hydrogen gas ($\text{H}_2$).

Count the atoms on both sides:
Reactants:
$\text{Mg}: 1$
$\text{H}: 1$
$\text{Cl}: 1$

Products:
$\text{Mg}: 1$
$\text{H}: 2$
$\text{Cl}: 2$

To balance the hydrogen and chlorine atoms, place a coefficient of 2 in front of $\text{HCl}$ on the reactant side.

Reactants:
$\text{Mg}: 1$
$\text{H}: 2$
$\text{Cl}: 2$

Products:
$\text{Mg}: 1$
$\text{H}: 2$
$\text{Cl}: 2$
The equation is now balanced.

Option A matches the balanced equation derived.

The final answer is $\text{A}$.