Which of the following types of malware is the MOST difficult to reverse engineer?

Recently, malware on a company computer destroyed several important files after it detected that Homer was no longer employed at the company. Which of the following BEST identifies this malware?

A recent antivirus scan on a server detected a Trojan. A technician removed the Trojan, but a security administrator expressed concern that unauthorized personnel might be able to access data on the server. The security administrator decided to check the server further. Of the following choices, what is the administrator MOST likely looking for on this server?