The correct answer is D. Integrity. In the CIA triad, integrity refers to protecting information from unauthorized alteration, modification, or revision, whether the data is stored at rest or moving across a network. CEH-aligned material explains that integrity ensures data received is exactly the same as the data originally sent, and any addition, deletion, or alteration means integrity has been compromised. The question states that an attack altered the contents of two critical files, which directly indicates unauthorized modification of data. This is an integrity violation because the trustworthiness, accuracy, and reliability of those files can no longer be guaranteed. Availability is incorrect because availability concerns whether systems or data are accessible when needed. Authentication is incorrect because it verifies the identity of a user, device, or process. Confidentiality is also incorrect because confidentiality focuses on secrecy and preventing unauthorized disclosure, not file modification. Therefore, an attack that changes critical file contents is best described as an integrity attack.