Below is a list of the response provided in the question:

${\mathrm{H}}_3{\mathrm{PO}}_4+\mathrm{Mg}(\mathrm{OH}{)}_2\to {\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2+{\mathrm{H}}_2\mathrm{O}$

We must now determine whether or not the reply is balanced. According to the law of conservation of mass, the mass of the reactants and products are equal. Furthermore, we are aware that mass cannot be generated or destroyed during a chemical interaction. To begin with, we'll make sure the reactants and products are accurate. Coefficients are employed in order to balance the response. To balance the reaction, we can only adjust the coefficients; the subscript close to each element cannot be altered.

As a result, if we check, there is one magnesium atom on the reaction's right side and three on its left. Similar to this, there are 6 oxygen atoms on the right side and 9 oxygen atoms on the left side of the reaction, respectively, and 5 hydrogen atoms on the right side and 2 hydrogen atoms on the left. Additionally, the reaction has 2 phosphorus atoms on the left and 1 sulphur atom on the right.

We will add three to the magnesium molecule on the right to bring it into equilibrium. We shall increase the hydrogen molecules on the right side, H₃PO₄, by two, Mg(OH)₂ by three, and on the left side, by six, in order to balance the hydrogen atom.
The chemical process is balanced when:

$2{\mathrm{H}}_3{\mathrm{PO}}_4+3\mathrm{Mg}(\mathrm{OH}{)}_2\to M{g}_3{\left({\mathrm{PO}}_4\right)}_2+6{\mathrm{H}}_2\mathrm{O}$

Hence option A is correct.