Explanation/Reference:
Explanation:
Cryptanalysis is the act of obtaining the plaintext or key from the ciphertext. Cryptanalysis is used to obtain valuable information and to pass on altered or fake messages in order to deceive the original intended recipient. This attempt at "cracking" the cipher is also known as an attack.
The following are example of some common attacks:
Brute Force. Trying every possible combination of key patterns - the longer the key length, the more

difficult it is to find the key with this method
Known Plaintext. The attacker has a copy of the plaintext corresponding to the ciphertext

Chosen Plaintext. Chosen plaintext is encrypted and the output ciphertext is obtained

Ciphertext Only. Only the ciphertext is available

Incorrect Answers:
B: A Brute Force attack involves trying every possible combination of key patterns. This is not what is described in the question.
C: With a Ciphertext Only attack, only the ciphertext is available. The plaintext is not available.
D: In a Chosen Plaintext attack, chosen plaintext is encrypted and the output ciphertext is obtained. This is not what is described in the question.
References:
Krutz, Ronald L. and Russel Dean Vines, The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, New York, 2001, p. 154