D

Calcium ions (factor IV), prothrombin (factor II) and fibrinogen (factor I) are core components of the coagulation cascade, whereas ferrous iron (Fe²⁺) plays no physiological role in clot formation and can even inhibit coagulation when present in excess [van Vliet et al., 1985, PMID 2864200; Rosenmund et al., 1984, PMID 6421970].