Answer:
d. furcula
Step-by-step explanation:
Furcula
Furcula is a synapomorphy linking dinosaurs and birds .
The furcula is the structure formed by midline fusion of clavicles and this very element is unique in the theropods as well . And is the most crucial for understanding the link between birds and the theropods .
The first suggestion was by Huxley in 1868 , and in 1870 , the key component of the debate is the presence of Frucula .