Which of the following is NOT a key recovery method?

A.
A secret key or a private key is broken into a number of parts and
each part is deposited with a trustee agent. The agents can then
provide their parts of the key to a central authority, when presented
with appropriate authorization. The key can then be reconstructed
and used to decrypt messages encrypted with that key.

B.
A message is encrypted with a session key and the session key is, in
turn, encrypted with the private key of a trustee agent. The
encrypted session key is sent along with the encrypted message. The
trustee, when authorized, can then decrypt the message by
recovering the session key with the trustee’s public key.

C.
A message is encrypted with a session key. The session key, in turn,
is broken into parts and each part is encrypted with the public key
of a different trustee agent. The encrypted parts of the session key
are sent along with the encrypted message. The trustees, when
authorized, can then decrypt their portion of the session key and
provide their respective parts of the session key to a central agent.
The central agent can then decrypt the message by reconstructing
the session key from the individual components.

D.
A message is encrypted with a session key and the session key is, in
turn, encrypted with the public key of a trustee agent. The
encrypted session key is sent along with the encrypted message. The
trustee, when authorized, can then decrypt the message by recovering
the session key with the trustee’s private key.

Explanation:
Encrypting parts of the session key with the private keys of the
trustee agents provides no security for the message since the
message can be decrypted by recovering the key components of
the session key using the public keys of the respective agents. These
public keys are available to anyone.
The other answers are valid means of recovering keys, since key recovery refers to permitting
access to encrypted messages under predefined circumstances.
Two of these answers are also called key encapsulation since the session
key is encapsulated in the public keys of the trustee agents and,
therefore, can be decrypted only by these trustee agents with their
private keys.