Explanation/Reference: https://blog.finjan.com/email-spoofing/

Threat hunting is a proactive process of searching for signs of malicious activity or compromise within a system or network, by using hypotheses, indicators of compromise, and analytical tools. Threat hunting can help improve detection capabilities by identifying unknown threats, uncovering gaps in security controls, and providing insights for remediation and prevention. Vulnerability scanning (A) is a reactive process of scanning systems or networks for known vulnerabilities or weaknesses that can be exploited by attackers. It can help identify and prioritize vulnerabilities, but not proactively hunt for threats. Red teaming is a simulated attack on a system or network by a group of ethical hackers who act as adversaries and try to breach security controls. It can help test the effectiveness of security defenses and response capabilities, but not proactively hunt for threats. Penetration testing (D) is similar to red teaming, but with a more defined scope and objective. It can help evaluate the security of a system or network by simulating real-world attacks and exploiting vulnerabilities, but not proactively hunt for threats.