(c) Diadelphous : The filaments are united forming two bundles, e.g., in Lathyrus $(sweet \,\,pea)$, Pisum $(edible\,\, pea)$ there are $10$ stamens;

filaments of $ 9 $ stamens are fused forming one bundle and the other bundle is represented by the filament of the tenth free stamen $ (A_1 + (9)).$