The main features of Umbrella for Cloud-Delivered Security include blocking malware, command and control (C2) callbacks, and phishing over any port or protocol. This comprehensive protection is essential for maintaining security across a range of internet activities and connections, making 'Blocks malware, C2 callbacks, & phishing over any port/protocol' the correct choice.

DNS-layer security primarily focuses on preventing threats and enforcing security policies based on DNS analysis. Live threat intelligence helps in identifying and mitigating threats as they occur, which is a core functionality of DNS-layer security. URL filtering is another core functionality, allowing the system to block or allow access to specific domains based on security policies. Proxy and file inspection, real-time sandboxing, and data analytics are more associated with broader security measures and not specifically confined to DNS-layer security.

Cyber Threat Defense & Network Analytics typically focus on securing cloud environments and endpoints. These services are designed to monitor traffic, detect anomalies, and prevent threats that can target various access points in a network, including the cloud platform and user end devices.

To secure IoT deployments, Cisco focuses on segmentation & visibility and cross-architecture automation. Segmentation & visibility helps in isolating IoT devices and providing insight into device behavior, thus enhancing security. Cross-architecture automation allows for consistent security policies and automated threat responses across different network architectures, which is crucial for a dynamic IoT environment.

Legacy networking approaches typically result in slow containment of security threats because these systems aren't optimized for rapid detection and mitigation. Additionally, they tend to create a large attack surface due to outdated infrastructure and protocols that can be easily exploited, increasing the vulnerability to attacks.