The correct answer is C. Leveraging DDoS Prevention Offerings from an ISP or DDoS Mitigation Service.

The scenario describes a large-scale DDoS attack that exceeds the organization’s local mitigation capacity. The traffic is rerouted through an upstream filtering or scrubbing infrastructure that absorbs malicious traffic and forwards legitimate traffic back to the organization. This is the function of an ISP-backed or specialized DDoS mitigation/scrubbing service.

CEH-aligned DDoS countermeasure material notes that, for a true DDoS attack, the practical answer is often involvement of the upstream ISP because endpoint-level defenses may not keep up with a sophisticated global botnet attack .

Option A. RFC 3704 Filtering is incorrect because ingress/egress filtering helps reduce spoofed traffic but does not describe large-scale traffic scrubbing.

Option B. Cisco IPS Source IP Reputation Filtering is incorrect because source reputation filtering alone does not match upstream traffic rerouting and scrubbing.

Option D. Black Hole Filtering is incorrect because blackholing drops traffic, often including legitimate traffic, while the scenario says clean traffic is forwarded back to the retailer.

Therefore, the best answer is C. Leveraging DDoS Prevention Offerings from an ISP or DDoS Mitigation Service.