Destination-based Remotely Triggered Black Hole (RTBH) routing mitigates Denial of Service (DoS) attacks by dropping malicious traffic at the network edge before it consumes vital bandwidth or device resources. In a destination-based black hole setup, traffic destined for the victim's IP address is routed to the Null0 interface (a virtual interface that acts as a discard route). According to the exhibit, the victims are the Web Server (10.20.10.2) and the Mail Server (10.30.10.2). To stop the attack at the ingress point (R1), strict /32 host routes for these specific server IPs must be statically routed to Null0.

B: Removing a route to 10.20.10.0 via the attacker's IP (10.10.10.2) is syntactically irrelevant to RTBH and does not blackhole traffic destined to the victim servers.

D: Similar to B, removing a route to the mail server subnet via the attacker's IP does not implement a destination-based drop and will not stop the attack.

E: This command routes traffic destined for R2's interface (10.20.10.1) to Null0, which leaves the actual Web Server (10.20.10.2) exposed to the attack.

Cisco Systems. "Remotely Triggered Black Hole Filtering—Destination Based and Source Based." Cisco IOS Security Configuration Guide, Securing the Data Plane. (Section: Destination-Based RTBH Filtering).

Internet Engineering Task Force (IETF). "RFC 5635: Remote Triggered Black Hole Filtering with Unicast Reverse Path Forwarding (uRPF)." (Section 3: Destination-Based RTBH). DOI: 10.17487/RFC5635.