Which combination of steps will meet these requirements?

Explanations:

Sharing the transit gateway with member accounts using AWS Resource Access Manager allows those accounts to attach their VPCs to the transit gateway, enabling interconnectivity across the organization.

AWS Organizations Service Control Policies (SCPs) are used for managing permissions within the organization but cannot be used to share a transit gateway, as they do not facilitate resource sharing or connectivity directly.

Launching a CloudFormation stack set from the management account to create a new VPC and a transit gateway attachment automates the setup process in member accounts and associates the attachment with the existing transit gateway.

While launching a CloudFormation stack set to create a new VPC is valid, using a peering transit gateway attachment does not meet the requirement, as the question specifies using a transit gateway attachment, not a peering option.

AWS Service Catalog is not used for sharing transit gateways between accounts. It is primarily for managing and deploying approved AWS resources, not for sharing connectivity resources like transit gateways.