The correct answer is D. Synsacrum.
A synsacrum is a fused mass of vertebrae that forms the posterior part of the pelvis in birds. It is formed by the fusion of the last few thoracic vertebrae, the sacral vertebrae, and the first few caudal vertebrae. The synsacrum provides a strong attachment for the hind limbs and helps to support the weight of the body.
The keel is a long, narrow bone that runs along the underside of the sternum in birds. It provides attachment for the flight muscles and helps to support the weight of the body during flight.
The furcula, or wishbone, is a forked bone that is formed by the fusion of the clavicles in birds. It helps to support the weight of the body and provides attachment for the muscles that move the wings.
A syncytium is a multinucleated cell that is formed by the fusion of multiple cells. It is found in some tissues, such as the placenta, and can also be formed in tumors.