Cisco Stealthwatch is a comprehensive, network telemetry-based, security monitoring and analytics solution that streamlines incident response through behavioral analysis; detecting denial of service attacks, anomalous behaviour, malicious activity and insider threats. Based on a scalable enterprise architecture, Stealthwatch provides near real-time situational awareness of all users and devices on the network.

Note: Although answer A seems to be correct but in fact, Stealthwatch does not provide real-time protection for DDoS attack. It just helps detect DDoS attack only.

Stealthwatch aggregates observed network activity and performs behavioral and policy driven analytics against what it sees in order to surface problematic activities. While we don't position our self as a DDOS solution, we're going to leverage our analytical capabilities to identify a DDoS attack against an internal host using the WebUI.