Which statement accurately describes the difference between plasma and serum?

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Difference in composition driven by whether the blood is allowed to clot. Plasma is obtained from blood collected with an anticoagulant, so it remains liquid and contains clotting factors, including fibrinogen. Serum comes from blood that is allowed to clot; after centrifugation, the liquid portion left behind lacks fibrinogen and other clotting factors because they are used up in forming the clot, and platelets become part of the clot rather than the liquid.

This makes the statement describing plasma as the liquid component with anticoagulant present and clotting factors, and serum as the liquid after clotting that lacks fibrinogen, the best description. The other options misstate what plasma or serum are (for example, calling plasma the solid portion or claiming serum contains platelets) or oversimplify the clotting-factor presence.