A large oil filter manufacturer for mid-size sedans has noticed an unusual amount of network traffic leaving their systems to unfamiliar IP addresses. Upon further investigation, they discover that a group of attackers has been using a combination of spear-phishing emails and malware to infiltrate their network and exfiltrate sensitive data. The attackers are highly skilled and have been able to evade detection by traditional security measures. What type of attack is this an example of?
a. Advanced persistent threat (APT)
b. Nation-state
c. Hacktivists
d. Organized crime