Final answer:
Ascorbic acid is not used as an anticoagulant, unlike heparin, EDTA, and ACD solutions, which are used in medical settings to prevent blood clotting.
Step-by-step explanation:
The substance among the options provided that is not used as an anticoagulant is ascorbic acid (vitamin C). Anticoagulants like heparin, EDTA (ethylenediaminetetraacetic acid), and ACD solution (acid citrate dextrose) are commonly used in medical and laboratory settings to prevent blood clotting. Heparin directly inhibits thrombin and other blood coagulation factors, while EDTA and ACD bind calcium ions preventing them from participating in the coagulation process. Ascorbic acid, however, does not have anticoagulant properties and is not used for this purpose.