Correct choice is (a) Alcohol dehydrogenase
To elaborate: Enzymes are widely used as in clinical lab as reagents for estimation of serum constituents. Alcohol dehydrogenase is used as a reagent in ethanol assay. Enzymes have been used as the label in various immunoassays for determining serum concentration of drugs, hormones or other compounds of interest. Commonly used enzymes as labels are: glucose-6-phosphate dehydrogenase, alkaline phosphatase, peroxidase and β-galactosidase.