Explanation
D: When duplicating a whole CDB or one more PDBs:
You must create the auxiliary instance as a CDB. To do so, start the instance with the following declaration in the initialization parameter file:
enable_pluggable_database=TRUE
A: To duplicate PDBs, you must create the auxiliary instance as a CDB. To do so, start the instance with the declaration enable_pluggable_database=TRUE in the initialization parameter file. When you duplicate one or more PDBs, RMAN also duplicates the root (CDB$ROOT) and the seed database (PDB$SEED). The resulting duplicate database is a fully functional CDB that contains the root, the seed database, and the duplicated PDBs.
https://docs.oracle.com/database/121/BRADV/rcmdupdb.htm

Explanation
Explanation:
When RMAN performs active database duplication using backup sets, a connection is established with the source database and the auxiliary database. The auxiliary database then connects to the source database through Oracle Net Services and retrieves the required database files from the source database. This method of active database duplication is also to as the pull-based method.
Example:
DUPLICATE TARGET DATABASE TO dupdb
FROM ACTIVE DATABASE
PASSWORD FILE
SPFILE
NOFILENAMECHECK;
Incorrect Answers:
A, B, D: When active database duplication is performed using image copies, after RMAN establishes a connection with the source database, the source database transfers the required database files to the auxiliary database. Using image copies may require additional resources on the source database. This method is referred to as the push-based method of active database duplication.
References: https://docs.oracle.com/database/121/BRADV/rcmdupdb.htm