You have a VPC with public and private subnets, and custom network ACLs are in use. Private subnet instances cannot reach the internet. An internet gateway is attached to the public subnet and the private subnet routes to a NAT gateway in the public subnet. EC2 instances use the VPC default security group. What is the most likely cause of the connectivity problem?

The most likely cause is a network ACL on the private subnet denying outbound traffic. Network ACLs are stateless, meaning if you deny inbound traffic, you must explicitly allow outbound traffic, and vice-versa. If the private subnet's network ACL is blocking outbound connections, instances cannot reach the NAT Gateway or the internet. Incorrect options: "There is no NAT gateway deployed in the private subnet" is incorrect because the question states the private subnet routes to a NAT gateway in the public subnet, which is the correct architecture. NAT gateways are typically in public subnets. "The VPC default security group is blocking all inbound traffic to the EC2 instances" is incorrect because the issue is outbound connectivity to the internet, not inbound access to the instances. "The VPC default security group is blocking all outbound traffic from the EC2 instances" is less likely than a NACL issue. Security groups are stateful and typically allow all outbound traffic by default unless explicitly restricted. Even if restricted, a NACL is a more common point of failure for subnet-wide connectivity issues.