The correct answer is A.
OBJ-1.1: An advanced persistent threat (APT) is a network attack in which an unauthorized person gains access to a network and stays there undetected for a long period of time. An APT attack intends to steal data rather than to cause damage to the network or organization. An APT refers to an adversary’s ongoing ability to compromise network security, obtain and maintain access, and use various tools and techniques. They are often supported and funded by nation-states or work directly for a nation-states’ government. Spear phishing is the fraudulent practice of sending emails ostensibly from a known or trusted sender to induce targeted individuals to reveal confidential information. An insider threat is a malicious threat to an organization from people within the organization, such as employees, former employees, contractors, or business associates, who have inside information concerning the organization’s security practices, data, and computer systems. Privilege escalation is the act of exploiting a bug, design flaw, or configuration oversight in an operating system or software application to gain elevated access to resources that are normally protected from an application or user. While an APT may use spear phishing, privilege escalation, or an insider threat to gain access to the system, the scenario presented in this question doesn’t specify what method was used. Therefore, APT is the best answer to select.