In the link to the AWS doc I would say that the answer is below the diagram. If traffic originates in Subnet B of VPC B it's the route table in VPC A that determines whether the traffic get's back to the correct sender.
Initially the answers is it will not because the route table in VPC A says "send traffic destined for 10.0.0.0/16 to VPC C".
You can't have both CidrIp and the GroupId in the Egress rule. One or the other. This works:
Do you really need Transit Gateway? As suggested in the answer to your other question VPC Peering may be enough. If not tell us why.
In that other question you confirmed that your setup in the end worked and you could connect from EC2 to RDS. If you can't now what has changed?
As suggested in the other answer and also by @Tim here in the comment do this:
i was using amazon EC2 many months without issues but recently i am getting this too. i am using Google public DNS as my DNS, not sure if it is related. i am also not sure if the problem began with my new internet connection or not, i guess it worked for a while even with new internet.
Btw i tried the public IP instead of domain name and no change.