During a controlled red team engagement at a financial institution in New Jersey, ethical hacker...

The correct answer is B. Stealth Virus because the defining characteristic described is hiding malicious presence by intercepting operating system calls and masking changes so that normal tools (including antivirus scans) do not observe the infection. In CEH-aligned malware concepts, stealth viruses are designed to evade detection by concealing modifications to files, boot records, or system areas. They commonly do this by hooking system functions or APIs so that when the OS or a security product requests file contents, sizes, checksums, directory listings, or other metadata, the virus returns clean-looking or original data instead of the infected/modified version. This makes infected files appear to “function normally,” while the malware remains active in memory and persists on disk.

The scenario explicitly mentions that “infected files continue to function normally” and that the malware “conceals its modifications by intercepting operating system calls.” That is the classic behavior of stealth techniques: manipulate what the system reports, not necessarily change the outward behavior of the application. The repeated “no threats detected” results also align: signature-based or basic scanning can be blinded when the malware controls the interface through which the scanner reads target files or system structures.

Why the other options are less correct: a polymorphic virus focuses on changing its code/signature between infections to evade signature-based detection, but the key clue here is OS call interception and hiding modifications, not code mutation. A macro virus targets macro-enabled documents and spreads through macro execution in office applications; it is not primarily defined by OS-level call hooking. A cavity virus (spacefiller) hides by inserting itself into unused areas of a file without changing the file size, but the scenario’s emphasis is on intercepting OS calls to conceal changes, which is more directly the stealth-virus behavior.

Therefore, Ryan most likely deployed a stealth virus.