Reaction rate measurement via concentration changes

Steps:

• Define reaction rate as the change in concentration of a substance per unit time.
• Identify that for reactants, rate is the negative change in concentration over time.
• Recognize that this directly quantifies how fast reactants are consumed.
• Confirm this matches standard chemical kinetics definitions.

Why A is correct:

• Reaction rate is formally defined as the decrease in reactant concentration per unit time, given by $r = -\frac{d[\text{reactant}]}{dt}$, allowing precise measurement.

Why the others are wrong:

• B measures product formation, but the standard rate definition uses reactant consumption, not product appearance.
• C tracks volume change, which indirectly relates to concentration via ideal gas law but is not a direct measure of reaction rate.

Final answer: A