D. It shows how many molecules there are of a particular substance.
In a chemical equation, a coefficient is placed in front of a chemical formula to indicate the number of molecules (or moles) of that substance involved in the reaction. For example, in the equation \(2H_2 + O_2 \rightarrow 2H_2O\), the coefficient "2" in front of \(H_2\) and \(H_2O\) indicates that there are two molecules of hydrogen gas and two molecules of water produced.