One of the most common systems used to classify information is the one developed within the US Department of Defense. These include: unclassified, sensitive, confidential, secret, and top secret.

The correct answer is "TFTP is less secure because session authentication does not occur". The Trivial File Transfer Protocol (TFTP) is considered less secure than the File
Transfer Protocol (FTP) because authentication does not occur during session establishment (although FTP is very insecure in its own right).

Explanation/Reference:
Explanation:
Accountability is the ability to identify users and to be able to track user actions. Through the use of audit logs and other tools the user actions are recorded and can be used at a later date to verify what actions were performed.
Incorrect Answers:
B: Common Criteria is an international standard to evaluate trust and would not be a factor in System Accountability.
C: Authorization is granting access to subjects, just because you have authorization does not hold the subject accountable for their actions.
D: Formal verification involves Validating and testing highly trusted systems. It does not, however, involve System Accountability.
References:
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, pp. 203, 248-250, 402.