Concept:Blood platelets are the formed elements that lack a nucleus.
Explanation:Blood platelets, also called thrombocytes, are not true cells but cell fragments.
They are produced by budding from megakaryocytes in the bone marrow.
Because they are fragments, platelets do not contain a nucleus.
In contrast, lymphocytes, monocytes, and eosinophils are white blood cells that all possess nuclei.
Mature mammalian red blood cells also lack a nucleus, but they are not listed among the options.
Therefore, the correct choice is blood platelets.
Answer:B. Blood Platelets