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When a blood specimen is allowed to clot, the correct outcome is serum plus blood cells. During the clotting process, fibrinogen, which is a protein found in plasma, is converted into fibrin strands that form a clot. This leads to the separation of the liquid portion of the blood, which becomes serum, from the cellular components.

Serum is essentially the fluid that remains after the clotting process has occurred and is devoid of clotting factors such as fibrinogen and prothrombin. This distinction is important because it indicates that while serum contains other substances such as electrolytes, hormones, and antibodies, it is not the same as plasma, which includes those clotting factors along with water, electrolytes, and proteins in a liquid state.

The terms used in the other choices do not accurately describe the outcome after clotting. Plasma, for instance, contains clotting factors and is obtained when blood is treated with anticoagulants to prevent clotting. Similarly, anti-coagulated blood will remain in a liquid state, and serum plus plasma combines two different outcomes of blood processing that do not occur simultaneously after clotting has taken place.