Which combination of steps will meet these requirements?

Explanations:

Sharing the transit gateway with member accounts using AWS Resource Access Manager (RAM) allows member accounts to access the transit gateway. This ensures connectivity between the VPCs in different accounts.

AWS Organizations Service Control Policies (SCPs) cannot be used to share or grant access to AWS resources like transit gateways. SCPs are used for controlling access to services, not for sharing resources.

Using AWS CloudFormation stack sets to automatically create a new VPC and a transit gateway attachment in each member account is a valid solution for automating this process. Associating the attachment with the transit gateway ensures that connectivity is established.

Peering transit gateway attachments are not the correct type of attachment for this use case. Transit gateway attachments should be of the type “VPC attachment,” not “peering.” Additionally, service-linked roles are not used for sharing attachments between accounts in this way.

AWS Service Catalog is used for managing portfolios of AWS resources and cannot directly be used to share a transit gateway. It’s not the correct tool for automating the attachment process in this scenario.