Explanation
According to the AWS documentation for Transit Gateway, a transit gateway is a network transit hub that connects VPCs and on-premises networks. A transit gateway route table is a set of rules that determines how traffic is routed among the attachments to the transit gateway1.
A transit gateway can have multiple route tables, and you can associate different attachments with different route tables. This allows you to control how traffic is routed between your VPCs and VPNs based on your network design and security requirements1.
The other options are incorrect because:
Both the TGW attachment and propagation must be in the same TGW route table is not true. You can associate an attachment with one route table and enable propagation from another attachment to a different route table. This allows you to separate the routing domains for your attachments1.
A TGW attachment can be associated with multiple TGW route tables is not true. You can only associate an attachment with one route table at a time. However, you can change the association at any time1.
The TGW default route table cannot be disabled is not true. You can disable the default route table by deleting all associations and propagations from it. However, you cannot delete the default route table itself1.
1: Transit Gateways - Amazon Virtual Private Cloud