Which of the following best defines a Trojan horse?

• A. A software program that contains various instructions that are carried out every time a computer is turned on
• B. A type of software that collects and reports information about a computer user without the user's knowledge or consent
• C. A virus that changes its structure to avoid detection
• D. A program or command procedure that gives the appearance that it is useful but in fact contains hidden malicious code that causes damage

Answer: (D)

A Trojan horse relies on deception: it disguises itself as something useful or legitimate, luring you to run it, but it secretly contains malicious code that performs damaging or unauthorized actions. Unlike a virus, it doesn’t replicate by itself to spread to other files or systems; it requires user action or another vulnerability to be introduced. It’s different from a program that just runs at startup, or from spyware that specifically focuses on covert data collection. The defining idea is the hidden payload paired with a credible appearance, which is why this option best defines a Trojan horse.