The correct answer is 'Trojan.' A Trojan is a type of malware that disguises itself as legitimate software. Attackers use Trojans to gain unauthorized access to systems by tricking users into loading and executing the malware on their systems. While the other options are also types of malware, they do not describe software that appears legitimate to gain unauthorized access. For instance, 'Ransomware' is designed to encrypt files and demand a ransom for the decryption key, 'Virus' attaches itself to legitimate software and spreads to other programs, and 'Spyware' typically aims to gather information without the user's knowledge. The key distinction for a Trojan is its disguise as legitimate software to perform malicious activities.