Which of the following is a computer program that appears to be legitimate but performs an illicit action when it is run?

A Trojan horse is a type of malicious software that deceives users by masquerading as a legitimate program. Users typically download or execute it, believing it provides a useful function, while in reality, it often carries out harmful actions, such as stealing sensitive data, installing additional malware, or creating backdoor access for attackers. This concept hinges on the element of deception; the program appears harmless or beneficial, which is what differentiates it from other types of malware.

Understanding the characteristics of a Trojan horse is crucial in recognizing the risks associated with malicious software. In contrast, options like redundant verification and parallel count pertain to data validation techniques and methods for ensuring data integrity, neither of which involves illicit actions. A web crawler, on the other hand, is a legitimate program used to index content from the internet, acting within legal and ethical boundaries. Therefore, the Trojan horse stands out as the correct choice due to its inherent deceptive nature and illicit purpose.