An Advanced Persistent Threat (APT) is the most probable threat actor in incidents involving the large-scale compromise of accounts in a hardened system. APTs are sophisticated, highly organized groups that employ advanced techniques to gain unauthorized access and remain undetected. Unlike poorly configured firewalls (A) or malware (B), which are often less targeted, APTs focus on strategic objectives, frequently using insider threats (D) or social engineering as part of their multi-vector attacks.