How would you complete and balance the acid base neutralizations reaction: $\mathrm{HBr}+ \mathrm{Ba}{\left(\mathrm{OH}\right)}_2\to$

According to the Law of Conservation of Mass, the mass of the reactants and the mass of the products must be equal. The atoms of all the elements and molecules on the reactant side (left side) and product side (right side) must be equal in order for a chemical equation to be balanced.
The chemical equation is balanced if the reactant and product sides both have an equal amount of atoms for each of the reacting elements. The equation must meet this condition in order to be in accordance with the law of conservation of matter.

The number of atoms represented in each formula for an element that appears in more than one on a given side of the equation must be calculated, and the total must then be totaled.
If the base in a neutralisation reaction is a hydroxide, then salt and water will be the end products.

When an acid reacts with a base, then the products obtained are salt and water, Thus, the complete equation of the acid-base reaction  between hydrogen bromide and Barium hydroxide is

$\mathrm{HBr}+ \mathrm{Ba}{\left(\mathrm{OH}\right)}_2\to {\mathrm{BaBr}}_2 + {\mathrm{H}}_2\mathrm{O}$

But the above equation is not a balanced equation as the number of Br, O, and H atoms are not the same on the reactant and product side. 

Thus, after balancing them, the balanced equation is-

$2\mathrm{HBr}+ \mathrm{Ba}{\left(\mathrm{OH}\right)}_2\to {\mathrm{BaBr}}_2 + 2{\mathrm{H}}_2\mathrm{O}$

Hence, the correct option is C).