Question

how is serum obtained? from clotted blood that has been centrifuged from whole blood that has been centrifuged by using a tube containing an anticoagulant from whole blood that was left standing

Explanation:

Serum is the liquid portion of blood after it has clotted. Clotted blood is centrifuged to separate the solid clot from the serum. Whole - blood with anticoagulant gives plasma, not serum. Letting whole blood stand doesn't yield serum in the standard way.

Answer:

A. From clotted blood that has been centrifuged