Final answer:
In a DWI case, the Breathalyzer is used to measure an individual's Blood Alcohol Concentration by detecting ethanol in their breath through a color change reaction.
Step-by-step explanation:
The piece of equipment used to determine BAC (Blood Alcohol Concentration) in a DWI case is a. Breathalyzer. A Breathalyzer is a device that measures the ethanol concentration in a person's breath. The chemical reaction involves the oxidation of ethanol by the dichromate ion, which is reflected in a color change of the test solution from yellow-orange to green. The intensity of the green color correlates with the amount of ethanol, and therefore, the BAC of the individual being tested.