The correct option is D Synsacrum
Synsacrum is the skeletal structure of birds in which the sacrum is extended by incorporation of additional fused or partially fused sacral or lumbar vertebrae. Synsacrum is formed by the fusion of last thoracic, lumbar, sacral and anterior few caudal vertebrae, e.g., in fowl 13 to 14 vertebrae are fused to form synsacrum.
Thus, the correct answer is (D), Synsacrum.