You are using Terraform to deploy a multi-tier application architecture consisting of a public subnet...

Problem Scope:Load balancer (public subnet) cannot reach application servers (private subnet).

Connectivity Flow:Load balancer initiates traffic to application servers; application servers respond. Key checkpoints: routing and security rules.

Analyze Routing:Private subnets typically don’t route to an Internet Gateway by default; they use NAT or Service Gateways. Misrouting (Option B) would affect outbound traffic, not inbound from the load balancer.

Security Rules:

Ingress (App Servers):Must allow traffic from the load balancer’s IP range.

Egress (Load Balancer):Must allow traffic to the application servers.

Evaluate Options:

A:Missing ingress rule on application servers’ security list blocks load balancer traffic; most likely.

B:Incorrect default route affects outbound, not inbound; less likely.

C:NAT misconfiguration impacts outbound, not inbound; incorrect.

D:Load balancer egress is necessary but secondary to application server ingress.

Conclusion:Ingress rule absence on the application server subnet is the primary blocker.

Security lists control traffic at the subnet level in OCI. The Oracle Networking Professional study guide explains, "For a load balancer in a public subnet to communicate with instances in a private subnet, the private subnet’s security list must include an ingress rule allowing traffic from the load balancer’s IP range" (OCI Networking Documentation, Section: Security Lists). Since Terraform deployed the infrastructure, a misconfigured security list is a common oversight.