For the common user:
The username must be unique across all containers. So D is correct because APP_USER1 is different from APP1_USER1 in APP1_ROOT B,E are wrong because you can only create local user in specific PDB.
A is true because APP1_USER1 is unique in PDB1
F is false because common user will not have the same privileges and role unless you grant it with option CONTAINER=ALL;