megakaryocytes are huge cells found in red bone marrow that extend long tendrils into blood sinusoids. The flow of blood breaks off small portions of those tendrils creating the formed element known as

Platelet

Explanation:

In 1906 James Homer Wright established that platelets, seen as hyaline bodies or vacuoles, are formed in the cytoplasm of megakaryocytes in the bone marrow, which then disintegrate and pass as platelets into the circulation. Megakaryocytes are cells of the myeloid lineage. They originate platelets (or thrombocytes) by cleaving parts of their enormous cytoplasm directly into a sinusoid vessel. The function of platelets is to form thrombi to prevent blood loss in small blood vessel ruptures.

looks like the answer is     c     to me!