What indicates how many atoms of each element are present in a chemical formula?

The correct choice is indicated by the subscript in a chemical formula, which plays a crucial role in specifying the number of atoms of each element in a compound. When a subscript follows a chemical symbol, it shows how many atoms of that particular element are present in the molecule. For example, in the chemical formula H2O, the subscript '2' next to hydrogen (H) indicates that there are two hydrogen atoms, while there is no subscript next to oxygen (O), implying that there is one oxygen atom.

Coefficients, on the other hand, are used in front of an entire chemical formula to indicate how many molecules or formula units of that compound are present, but they do not specify the number of atoms of each element within a single molecule. Z-index does not relate to chemistry and is not a relevant term in this context. A numerical value can be ambiguous and does not specifically denote the number of atoms in terms of chemical structure. Thus, the subscript is the essential feature that directly provides the count of individual atoms for each element in a chemical formula.