Conversion of sulfur dioxide to sulfuric acid
A. $2{\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}} \to 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} + \left[ {\text{H}} \right]$
B. \[{\text{S}}{{\text{O}}_2} + 4{{\text{H}}_2}{\text{O}} \to 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} + 2\left[ {\text{H}} \right]\]
C. ${\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}} \to {{\text{H}}_2}{\text{S}}{{\text{O}}_4} + 2\left[ {\text{H}} \right]$
D. $2{\text{S}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}} \to 2{{\text{H}}_2}{\text{S}}{{\text{O}}_4} + 2\left[ {\text{H}} \right]$

Hint: Chemical reaction is the process by which a chemical change takes place. Chemical equation is a shorthand way of expressing chemical reactions using numbers. For balancing chemical equations, we have to follow the law of conservation of mass.

Complete step by step answer:
A chemical equation is a symbolic representation of a chemical reaction. The reactant chemicals remain on the left side of the equation and the product chemicals are on the right side of the equation. The two sides are connected with an arrow leading from left to right. A balanced chemical equation represents the chemical reaction using chemical formulae of reactants and molecules. It indicates the number of molecules or atoms of each substance. The rules for balancing chemical equation are:
Add coefficients in front of a molecule.
Never use subscripts or parenthesis other than coefficients.
Adjust the coefficients not the subscripts.
A. Number of each element in the reactant side will be equal to the number of elements in the product side. But in this option, the number of oxygen atoms is six in the reactant side while eight in the product side.
This does not follow the law of conservation of mass.
B. Here, the number of sulfur in the reactant side differs from that in the product side. So it cannot be considered.
C. This equation follows the law of conservation of mass. Number of elements in the reactant side is equal to the number of elements in the product side.
Hence option C is correct.

Additional information:
Chemical reactions help to determine the stoichiometry calculations by understanding the ratio between reactants and products. We have to remember that the mass is neither created nor destroyed.

Note:
Balancing can also be done in another method. First write the number of atoms per element in reactant and product sides. Fix one element and balance another element. On further fixing and balancing, we can equate the chemical equation.