Final answer:
Chemiluminescent enzyme labels often used in indirect procedures are alkaline phosphatase, horseradish peroxidase, and beta-galactosidase, meaning the correct answer is 'All of the above'.
Step-by-step explanation:
Chemiluminescent enzyme labels commonly used in indirect procedures, such as enzyme immunoassays (EIAs), include alkaline phosphatase, horseradish peroxidase, and beta-galactosidase. These enzymes are used for tagging antibodies in immunoassays, where the targeted binding of these antibodies to specific antigens can be visualized and quantified through the enzymatic conversion of a substrate. The most widespread enzymes are alkaline phosphatase and horseradish peroxidase due to the availability of suitable substrates that these enzymes convert into either a colored or fluorescent product, depending on the design of the assay. Beta-galactosidase is also used, although less commonly than the other two. Therefore, the correct answer to which chemiluminescent enzyme labels are often used in indirect procedures is D. All of the above.