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

Explanation:
To configure a custom application with Cisco SD-WAN centralized policy, you need to follow these steps25:
Click Configuration, select Policies, and then select Centralized Policy.
Click Custom Options, select Centralized Policy, and then select Lists.
Click Custom Applications, and then select New Custom Application.
Enter a name for the application, enter the match criteria, and then click Add.
The process of configuring a custom application with a Cisco SD-WAN centralized policy using Cisco vManage involves several steps1.
Click Configuration, select Policies, and then select Centralized Policy: This is the first step where you navigate to the Policies section in the Configuration menu of Cisco vManage1.
Click Custom Options, select Centralized Policy, and then select Lists: In this step, you select the Custom Options, then select Centralized Policy, and finally select Lists1.
Click Custom Applications, and then select New Custom Application: After setting up the Lists, you click on Custom Applications and then select New Custom Application1.
Enter a name for the application, enter the match criteria, and then click Add: Finally, you enter a name for the application, specify the match criteria, and then click Add to complete the configuration1.
References :=
Cisco Catalyst SD-WAN Policies Configuration Guide, Cisco IOS XE

To redistribute OSPF internal routes into BGP for connecting an on-premises network to a cloud provider, the engineer should run the commands "router bgp 100" and "redistribute ospf 1" on router R2. The command
"router bgp 100" is used to create a BGP routing process with AS number 100. The command "redistribute ospf 1" is used to redistribute OSPF routes from process ID 1 into BGP. References: = I need to access the specific content of Designing and Implementing Cloud Connectivity (ENCC) v1.0 from Cisco's official resources to provide exact references. However, I don't have direct access to external databases or resources, including the Cisco ENCC course materials. I recommend referring to the ENCC course materials for the most accurate and detailed information. Please note that this answer is based on general networking principles and may not reflect the specific content of the ENCC course. Always refer to the official course materials for the most accurate information.