Final answer:
In the chemical equation, option 2) B identifies a subscript, which indicates the number of atoms of an element in a molecule. Subscripts are part of the chemical formula and cannot be changed during equation balancing, as opposed to coefficients.
Step-by-step explanation:
The letter that identifies a subscript in the chemical equation is option 2) B. Subscripts in a chemical formula represent the number of atoms of the element that are contained within a molecule. For example, in the molecule AB4, the subscript '4' after the B indicates that there are four B atoms covalently bonded to the central A atom. It's important to note that subscripts are a fundamental part of a chemical formula and cannot be changed when balancing chemical equations.
In contrast, coefficients are numbers placed in front of the chemical formulas to indicate the quantity of molecules, and unlike subscripts, coefficients can be adjusted to balance a chemical equation.
Remember, in chemical equations, coefficients indicate the number of molecules and can be changed to balance the equation, while the subscripts indicate the number of atoms within a molecule and must remain constant.