Answer:
The correct answer is option a. "coefficient".
Step-by-step explanation:
In balanced chemical equations, coefficient are written down in front of chemical formulas representing the ratio of each substance. For instance, in the formula of water H2 + O2 → 2 H2O, the "2" represents the coefficient of the substance H2O. If a substance does not have a coefficient written down in front of it, it is assumed that it is equal to "1".