To diagnose why the Amazon SQS queue is not receiving messages from an EC2 instance in a private subnet, we need to consider the permissions and network configurations that could be affecting the communication between the EC2 instance and the SQS service. Option A is a possible cause because if the EC2 instance does not have an IAM role attached that grants it the necessary permissions to perform write operations on the SQS queue, it won't be able to send messages. Option C is another possible cause because without an interface VPC endpoint for SQS, the EC2 instance in a private subnet would need to route traffic through the internet to reach SQS, which is not possible in this scenario since there's no public subnet or internet gateway. Options B, D, and E are less likely to be the cause because the default security group allows all outbound traffic, the default network ACL allows all inbound and outbound traffic, and the route table would inherently have a route for the VPC's CIDR block, which includes the SQS service's IP address range if an interface VPC endpoint is used.