A company has just experienced a malware attack affecting a large number of desktop users. The antivirus solution was not able to block the malware, but the HIDS alerted to C2 calls as 'Troj. Generic'. Once the security team found a solution to remove the malware, they were able to remove the malware files successfully, and the HIDS stopped alerting. The next morning, however, the HIDS once again started alerting on the same desktops, and the security team discovered the files were back. Which of the following BEST describes the type of malware infecting this company's network? a. Trojan
b. Spyware
c. Rootkit
d. Botnet