Answer: Rootkit
Explanation: Rootkit is the software that helps in carrying out malicious attack by giving the permission to unauthorized user for accessing the system. This software comes with numerous tools for attacking the restricted and confidential information and resources of the system such as password stealers, key-loggers , antivirus disabling tool etc.
Network defender can lose trust when rootkit software gets into the operating system because it gives complete control for privileged accessing the system to the attacker .The detection and elimination of rootkit is also a difficult task.
Other options are incorrect because worm is malware that copies and spreads from one computer system to other. Trojan provides the unauthorized access of attacker in the system and can infect system but cannot be highly harmful as rootkit. Exploit is a code that enters the system to provide access to cyber-criminals but it can be detected .
Thus ,the correct option is rootkit.
