A server has been infected by a certain type of Trojan. The hacker intended to utilize it to send and host junk mails. What type of Trojan did the hacker use?

Turtle Trojans is not a recognized or standard classification in Trojan malware taxonomy.

Ransomware Trojans are designed to encrypt victim files and demand payment for decryption, not to send or host spam mail.

A Botnet Trojan compromises a system and enrolls it into a network of infected machines controlled by a command-and-control server. This infrastructure is commonly exploited to send bulk junk/spam email and host malicious or illegal content. The distributed nature of botnets makes them ideal for this type of high-volume, covert activity.

Banking Trojans are specifically engineered to steal financial credentials and intercept banking sessions, not to facilitate junk mail campaigns.