A Trojan horse refers to a computer program that...

A Trojan horse is a type of malicious software designed to appear harmless or beneficial while actually performing harmful actions in the background without the user's knowledge. The correct answer highlights that a Trojan horse opens a backdoor on a machine, which allows an attacker to gain remote access to the system.

Once this backdoor is established, the attacker can manipulate the machine, extract sensitive data, install additional malware, or leverage the machine for other malicious activities. Unlike viruses or worms, which self-replicate and spread automatically, a Trojan relies on tricking the user into downloading and executing it, often masked as legitimate software. This deceptive nature is central to the function of a Trojan horse.

The other options do not accurately describe the primary function of a Trojan horse. For example, while some malware can slow down a machine by overloading its resources, that is not the defining characteristic of a Trojan. Similarly, the third option suggesting that a Trojan creates protection against viruses is misleading, as Trojans are inherently harmful rather than protective. Thus, the essence of a Trojan horse is indeed its ability to create vulnerabilities by allowing unauthorized access, aligning perfectly with the correct answer.