Which of the following encrypts its code differently with each infection or generation of infections?

A boot sector virus infects the master boot record or volume boot record but does not change or re-encrypt its own code between infections.

A stealth virus intercepts system calls to hide its presence from the OS and AV tools but does not modify its own code encryption per infection.

A polymorphic virus uses a built-in mutation engine to re-encrypt or alter its code signature with every new infection or generation. This means each copy of the virus looks different to antivirus scanners, preventing simple hash or signature matching. It is distinct from a metamorphic virus, which rewrites its entire body, but both share the goal of evading static detection.

A macro virus embeds itself in document macros and spreads when infected documents are opened but uses a fixed code body, not per-infection encryption.