Which statement is true concerning dropping a pluggable database (PDB)? 

Answer options:

A. The PDB must be open in read-only mode.
B. The PDB must be in mount state.
C. The PDB must be unplugged.
D. The PDB data files are always removed from disk.
E. A dropped PDB can never be plugged back into a multitenant container database (CDB).

B

http://docs.oracle.com/database/121/ADMIN/cdb_plug.htm#ADMIN13658