The correct answer is A.
OBJ-1.5: An insider threat is any current or former employee, contractor, or business partner who has or had authorized access to an organization’s network, system, or data and intentionally exceeded or misused that access in a manner that negatively affected the confidentiality, integrity, or availability of the organization’s information or information systems. Based on the details provided in the question, it appears the employee’s legitimate credentials were used to conduct the breach. This would be classified as an insider threat. A zero-day is a vulnerability in software unpatched by the developer or an attack that exploits such a vulnerability. A known threat is a threat that can be identified using a basic signature or pattern matching. An advanced persistent threat (APT) is an attacker with the ability to obtain, maintain, and diversify access to network systems using exploits and malware.