Which of the following is the primary objective of a rootkit?

Opening a port to provide unauthorized services is the behavior of a backdoor or remote access trojan (RAT), not the defining characteristic of a rootkit.

Creating a buffer overflow is an exploit technique used to gain initial access or escalate privileges - it is a means of attack delivery, not a rootkit's objective.

Rootkits achieve their core goal - hiding attacker activity - by replacing legitimate OS programs (such as ls, ps, netstat, and login) with modified versions that suppress any output revealing malicious processes, files, or connections. This systematic replacement of trusted system executables is what defines a rootkit and separates it from backdoors or exploit code.

Providing an undocumented opening in a program describes a backdoor, which may be installed by a rootkit as a secondary payload but is not the rootkit's primary objective.