Final Answer:
Serum can't coagulate because the fibrinogen has been removed.
Option B is the answer.
Step-by-step explanation:
Coagulation is the process of blood clotting, which involves the conversion of fibrinogen into fibrin threads to form a stable blood clot. Serum is the liquid portion of blood that remains after coagulation, and it lacks clotting factors, including fibrinogen. When blood clots, the fibrinogen is converted into fibrin, and the remaining liquid is serum. Therefore, serum cannot coagulate because the fibrinogen, which is essential for clot formation, has already been removed during the clotting process.
Platelets, red blood cells, and white blood cells are cellular components of blood, and their presence or absence does not directly affect the coagulation of serum. Fibrinogen, a plasma protein, plays a central role in the coagulation cascade.
Option B is the answer.