A (not B,not E): The CREATE DATABASE ... ENABLE PLUGGABLE DATABASE SQL statement creates a new CDB. If you do not specify the ENABLE PLUGGABLE DATABASE clause, then the newly created database is a non-CDB and can never contain PDBs.
D: You can create a PDB by plugging in a Non-CDB as a PDB. The following graphic depicts the options for creating a PDB:

Description of cncpt358.png follows
Incorrect:
Not E:For the duration of its existence, a database is either a CDB or a non-CDB. You cannot transform a non-CDB into a CDB or vice versa. You must define a database as a CDB at creation, and then create PDBs within this CDB.