What is the correct sequence which enables an authorized agency to use
the Law Enforcement Access Field (LEAF) to decrypt a message sent by
using the Clipper Chip? The following designations are used for the
respective keys involved Kf, the family key; Ks, the session key; U, a
unique identifier for each Clipper Chip and Ku, the unit key that is
unique to each Clipper Chip.

A.
Decrypt the LEAF with the family key, Kf; recover U; obtain a court
order to obtain Ks, the session key. Use the session key to decrypt the
message.

B.
Decrypt the LEAF with the family key, Kf; recover U; obtain a court
order to obtain the two halves of Ku; recover Ku; and then recover Ks,
the session key. Use the session key to decrypt the message.

C.
Obtain a court order to acquire the family key, Kf; recover U and Ku;
then recover Ks, the session key. Use the session key to decrypt the
message.

D.
Obtain a court order to acquire the two halves of Ku, the unit key.
Recover Ku. Decrypt the LEAF with Ku and then recover Ks, the
session key. Use the session key to decrypt the message.

Explanation:
The explanation is based on the LEAF as shown in the Figure.

The message is encrypted with the symmetric session key, Ks. In order to
decrypt the message, then, Ks must be recovered. The LEAF contains
the session key, but the LEAF is encrypted with the family key, Kf ,
that is common to all Clipper Chips. The authorized agency has
access to Kf and decrypts the LEAf. However, the session key is still
encrypted by the 80-bit unit key, Ku, that is unique to each Clipper
Chip and is identified by the unique identifier, U. Ku is divided into
two halves, and each half is deposited with an escrow agency. The
law enforcement agency obtains the two halves of Ku by presenting
the escrow agencies with a court order for the key identified by U.
The two halves of the key obtained by the court order are XORed
together to obtain Ku. Then, Ku is used to recover the session key, Ks,
and Ks is used to decrypt the message.
The decryption sequence to obtain Ks can be summarized as:

This is the sequence described in answer “Decrypt the LEAF with the family key, Kf; recover U;
obtain a court order to obtain the two halves of Ku; recover Ku; and then recover Ks,
the session key. Use the session key to decrypt the message”. The sequences
described in the other answers are incorrect.