Step 1: Heparin's Function.
Heparin is an anticoagulant, preventing blood clot formation by deactivating thrombin and other clotting factors. In a heparinized tube, blood remains liquid post-collection, even after standing or centrifugation.
Step 2: Centrifugation of Blood.
Centrifugation separates blood components by density. Red blood cells (RBCs) settle at the bottom. A buffy coat layer, containing leukocytes (WBCs) and platelets, forms in the middle. Plasma, the lightest component, accumulates at the top.
Step 3: Plasma vs. Serum Distinction.
Plasma is the liquid portion of unclotted blood, containing fibrinogen and clotting factors. Serum is derived from clotted blood. As heparin inhibits clotting, the liquid separated from heparinized blood is plasma.
Step 4: Outcome of Centrifugation.
Centrifuging heparinized blood results in the separation of plasma, the buffy coat, and RBCs.
Final Answer:
\[\boxed{\text{The correct answer is (A) Plasma, buffy coat, RBC.}}\]