A virus typically requires which of the following to operate?

A virus typically attaches itself to an existing program in order to execute and spread. This is because viruses are designed to embed themselves within host files or applications, often waiting for the host program to run before they activate. By doing this, they can leverage the existing program's functionality, making them more likely to remain undetected while they replicate or carry out their malicious activities.

The other options, while they relate to how viruses might be spread or their operational environment, are not requirements for the virus itself to function. Viruses do not inherently need a specific operating system or email client; they can propagate across different systems if they have the means to infect appropriate host programs. Special user permissions can certainly facilitate the execution of malicious code but are not strictly necessary for a virus to operate.