A stealth network attack in which an unauthorized person gains access to a network and remains undetected for a long time is referred to as a(n) attack.

"Advanced persistent threat " is the right one.

Explanation:

APT seems to be a clandestine cyber assault on something like a network connection where another assailant achieves and retains unauthorized knowledge to the communication network, however impossible to conceive for a considerable period. They represent compound channel assaults that use different components including various approaches of attack.