What sub-component secures the bayonet on the upper receiver assembly?

The bayonet stud is the specific sub-component that secures the bayonet on the upper receiver assembly of the M16A3 and M4A1 rifles. Its design allows for a secure connection, ensuring that the bayonet remains firmly in place during use. When attached, the bayonet stud provides stability and strength, making it integral for both practical combat situations and ceremonial uses.

In contrast, while the carry handle is part of the upper receiver and serves to assist in aiming and carrying the rifle, it does not play any role in attaching the bayonet. The ejection port cover protects the ejection port from dirt and debris but is unrelated to bayonet attachment. Lastly, the charging handle is essential for chambering rounds and clearing jams but does not interact with the bayonet. Therefore, the bayonet stud is the correct answer as it is the designated component for securing the bayonet.