In the given scenario, an engineer is configuring a site-to-site IPsec VPN connection between an on-premises Cisco IOS XE router and AWS. The correct command to complete the tunnel configuration is “tunnel source 20.20.20.21”. This command specifies the source IP address for the tunnel, which is essential for establishing a secure connection between two endpoints over the internet or another network1. References:

Configure IOS-XE Site-to-Site VPN Connection to Amazon Web Services - Cisco Community

[Security for VPNs with IPsec Configuration Guide, Cisco IOS XE Release 3S - Config