An IT security analyst at a mid$-$sized company has observed unusual network activity on a workstation over the past few days. This workstation has initiated frequent and unsolicited communications with an unknown external IP address. Further investigation reveals the presence of unauthorized software on the workstation, which seems to be actively transmitting sensitive system data to this external address and possibly receiving commands or files in return without any visible signs or knowledge of the user. Given these specific behaviors, what type of malware is MOST likely responsible for these activities?

A$.$ Potentially Unwanted Program $($PUP$)$

B$.$ Virus

C$.$ Rootkit

D$.$ Remote Access Trojan $($RAT$)$