Final answer:
The purple top tube contains an anticoagulant that works by binding calcium, and it is mainly used for hematological assays including CBC.
Step-by-step explanation:
The tube that contains an anticoagulant that works by binding calcium is d) Purple top. Purple top tubes, also known as EDTA tubes, contain the anticoagulant Ethylenediaminetetraacetic acid (EDTA). EDTA acts by chelating (binding) calcium ions, which are necessary for the clotting cascade. Without calcium, the clotting process is inhibited, ensuring that blood remains in a fluid state for testing purposes such as complete blood count (CBC) and other hematological assays.