The correct answer is Nation-state, as these types of threat actors are known for having a high level of sophistication and capability. They have the resources to develop and use custom malware and elaborate social engineering tactics. Organized crime groups also have resources but typically engage in financial gain activities through less sophisticated means like ransomware or financial frauds. While insider threats can be sophisticated, the use of custom, advanced malware points more towards an external actor with significant resources. Unskilled attackers lack the knowledge and resources to create and deploy custom, sophisticated malware.