What type of virus is described as one that loads itself onto the target system's memory, infects other files, and then unloads itself?

The type of virus that is described as one that loads itself onto the target system's memory, infects other files, and then unloads itself is indeed a direct-action virus. This type of virus operates by first executing and residing in the system's memory. It then targets specific files to infect them—typically program files—before it unloads itself from the memory. The key characteristic of a direct-action virus is this temporary presence in memory, allowing it to evade some forms of detection during its operation.

In contrast, boot sector viruses are designed to infect the master boot record of a storage device and are activated when the system starts up, which is a different mechanism altogether. Network viruses exploit network connections to spread from one computer to another and are often designed to propagate over networked systems rather than focusing on direct manipulation of memory and file systems. Thus, the definition of a direct-action virus aligns perfectly with how it operates, confirming it as the correct answer.