Reference Material:

https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/high_availability_origin_failover.html
A. Wrong, one bucket has been added as origin already.
B. We're not using Route 53.
C. We're not using Route 53.
D. Correct, we have to add the new bucket as an origin.
E. We setup a CloudFront origin group with us-east-1 bucket and ap-southeast-1 as the secondary.
https://blogs.mulesoft.com/wp-content/uploads/EIrexPZKUz3z-866x1024.png
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/high_availability_origin_failover.html
https://docs.aws.amazon.com/cloudfront/latest/APIReference/API_OriginGroup.html:
"An origin group includes two origins (a primary origin and a second origin to failover to) and a failover criteria that you specify"
This supports E
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/high_availability_origin_failover.html:
"You can set up CloudFront with origin failover for scenarios that require high availability. To get started, you create an origin group with two origins: a primary and a secondary. If the primary origin is unavailable, or returns specific HTTP response status codes that indicate a failure, CloudFront automatically switches to the secondary origin.
To set up origin failover, you must have a distribution with at least two origins. Next, you create an origin group for your distribution that includes two origins, setting one as the primary. "
This supports D