Two VPCs are connected through Transit Gateway. Traffic from AZ A works, but resources in AZ B cannot reach the gateway. Which implementation detail should be checked first?

1. Create one attachment subnet only and rely on DNS to provide cross-AZ transit.
2. Create an internet-facing ALB for all non-HTTP packets.
3. Include an attachment subnet in each AZ that needs to send traffic through the Transit Gateway.
4. Give every instance a public IP address to reach the transit hub.