Answer:
The symbol in the parenthesis indicate (C) the physical state of each reactant and product
Step-by-step explanation:
There are symbols used to represent the physical state of a reactant or product, the most common being (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous, or water like. Precipitate is also considered a physical state and is represented by a downward-facing arrow. In this case, the only symbols used in the chemical equation are (aq) and (s). Therefore, C is the correct answer.
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The other answers:
A. the catalyst used in the reaction
Because it is neither a reactant nor a product, a catalyst is shown in a chemical equation by being written above the yield arrow. The yield arrow is what would be the equals sign in a normal equation.
B. the number of atoms of each reactant and product
These are represented by the subscripts. In the given chemical equation, these numbers are 2, 3, 3, and 2 (left to right).
D. the number of units of each reactant and product
These are represented by the coefficients. In the given chemical equation, these numbers are 1, 2, 1, and 2 (left to right). The ones are implied, so there's no need to write them down.
The other explanation did not include the multiple choice answers, so I hope this helps those who need it!