Which element protects and hides an internal network in an outbound flow?

A. DNS sinkholing: DNS sinkholing redirects DNS requests for known malicious domains to a designated server, preventing users from accessing those sites. It doesn't inherently protect or hide an internal network in outbound flows. It's more of a preventative measure against accessing malicious external resources.

B. User-ID: User-ID maps network traffic to specific users, enabling policy enforcement based on user identity. It provides visibility and control but doesn't hide the internal network's addressing scheme in outbound connections.

C. App-ID: App-ID identifies applications traversing the network, allowing for application-based policy enforcement. Like User-ID, it doesn't mask the internal network's addressing.

D. NAT (Network Address Translation): NAT translates private IP addresses used within an internal network to a public IP address when traffic leaves the network. This effectively hides the internal IP addressing scheme from the external network. Outbound connections appear to originate from the public IP address of the NAT device (typically the firewall), thus protecting and hiding the internal network's structure.