Final answer:
Eavesdropping is the category of attack primarily against confidentiality, as it involves the unauthorized interception of communications.
Step-by-step explanation:
The category of attack that is primarily against confidentiality is Eavesdropping (D). Eavesdropping involves unauthorized interception of private communications. It is a passive attack that is intended to access and potentially steal data that is being transmitted between two parties. Other options such as Malware can compromise confidentiality but it is not its primary target. Malware is often designed to cause damage or disruption. A Denial of Service (DoS) attack aims to make a system unavailable to its intended users, mostly affecting availability. Phishing involves the use of deceptive communications to trick individuals into revealing sensitive information, which does target confidentiality but more within the context of manipulation and deceit.