Which software security feature is supported by the Cisco ISR 4451 router?

IPsec/GRE cloud proxy is not a named security feature of the ISR 4451 - the router supports IPsec and GRE tunnels independently but not as a combined cloud proxy service.

Reverse-proxy is a web application feature typically found on dedicated load balancers or application delivery controllers, not a supported software security feature on the ISR 4451.

The Cisco ISR 4451 natively supports Zone-Based Policy Firewall (ZBF) with Application Visibility and Control (AVC), which together constitute the Enterprise Firewall with Application Awareness feature. This is a software-based security service that can be enabled on the platform without additional hardware. It inspects application-layer traffic and enforces per-application security policies.

Cloud Express service is a Cisco Express Forwarding-related optimization service, not a software security feature offered on the ISR 4451 platform.