A. Log in to the database virtual machine as the root user, use ssh-keygento generate four new keys, and assign them to each user in their home directory.

B. As the opcuser add additional public keys to the .ssh/authorized_keys file.

C. From the Compute Cloud > Network tab, access the security rules and enable the ssh_multiple_accessrule. Then you will be able to assign multiple keys to an instance through the Compute Cloud admin utility.

D. According to Oracle documentation, only a single key is supported per instance. Adding more users with their own public keys is not permissible.

E. Add additional keys using the 'Add New Key' menu in the Oracle Database Cloud Service console.