A security consultant decides to use multiple layers of anti-virus defense, such as end user desktop anti- virus and E-mail gateway. This approach can be used to mitigate which kind of attack?

A forensic attack is not a standard offensive attack category - forensics is an investigative discipline, not an attack type that AV layers would mitigate.

ARP spoofing is a Layer 2 network-level attack that poisons the ARP cache to redirect traffic; anti-virus software on desktops and email gateways does not inspect or prevent ARP-level activity.

Social engineering attacks such as phishing frequently rely on email-borne malware or malicious attachments to compromise users. Deploying anti-virus at both the email gateway (to block malicious messages before delivery) and the end-user desktop (as a last line of defense) implements defense-in-depth that addresses the primary delivery vector for these attacks.

Scanning attacks (port or vulnerability scanning) are reconnaissance activities at the network layer; anti-virus solutions do not detect or block network scan traffic.