References: RFC 2459 : Internet X.509 Public Key Infrastructure Certificate and CRL Profile; GUTMANN, P., X.509 style guide.

This problem is analogous to asking the question How many
people must be in a room for the probability of two people having
the same birthday to be equal to 50%? The answer is 23. Thus,
trying 2m/2 possible trial inputs to a hash function gives a 50%
chance of finding two inputs that have the same hash value. Answer
a, describes an attack in which the attacker can choose the plaintext
to be encrypted and can modify his/her choice based on the results
of a previous encryption.
* Answer the chosen-cipher text attack is where the attacker can select different ciphertexts to be decrypted and has the decrypted plaintext available. This attack is used to determine the key or keys being used. Answer d is an attack against double encryption. This approach shows that for a key length of k
bits, a chosen-plaintext attack could find the key after 2k+1 trials
instead of 22k attempts. In this attack on double encryption, one
encrypts from one end, decrypts from the other and compares the
results in-the-middle.

Explanation/Reference:
Explanation:
The concealment cipher is a symmetric key, transposition cipher where the words or characters of the plaintext message are embedded in a page of words or characters at a consistent interval.
Incorrect Answers:
A: Transposition cyphers moves the original values around.
C: The substitution cipher substitutes bits, characters, or blocks of characters with different bits, characters, or blocks.
D: Steganography is a technique used to hide data in another media type so that the presence of the data is masked.
Reference:
Miller, David R, Microsoft CISSP Training Kit, O'Reilly Media, 2013, California, p. 156 Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, pp. 774, 777