Explanation:

- Trap doors: Known as backdoors, these are secret entry points used for unauthorized access, often intentionally left by developers. They are not independent programs.

- Trojan horse: Disguised as legitimate software, these programs trick users into installing them. They rely on other software to spread.

- Virus: These malicious programs attach themselves to files and require a host to spread. They cannot work independently.

- Worm: Worms are self-replicating programs that spread across networks without needing a host.

- None of these: This option is incorrect because an independent malicious program does exist in the list.