Final answer:
A Trojan horse commonly opens a back door for malicious software, allowing unauthorized access to a computer system. They are named after the Trojan horse of legend because they disguise themselves as harmless software to execute their malicious activities.
Step-by-step explanation:
Trojan horses are a type of malicious software (malware) that can perform a variety of actions on a victim's computer without their knowledge. One of the common actions a Trojan horse might do is A) open a back door for malicious software. This allows attackers to gain unauthorized access to the affected computer, often without the user's knowledge. This back door can be used to control the system, steal sensitive information, or install more malware. The other options, such as changing memory configurations, altering ports, or changing an IP address, are not typical behaviors associated with Trojan horses.
Another important aspect of Trojan horses is their deceptive nature. They often disguise themselves as legitimate software, which is why they are compared to the historical Trojan horse; they seem harmless until their malicious functions are activated. It's worth noting that while Trojans might not directly modify memory configurations or IP addresses, the back doors they create could potentially enable attackers to perform those actions remotely if they choose.