Which factors could cause the health check failures?

Explanations:

The target instance’s security group must allow incoming traffic from the Network Load Balancer (NLB) for health checks to succeed. If the security group is blocking traffic from the NLB, the health checks will fail, preventing the instance from entering the InService state.

Security groups are associated with instances, not with load balancers. The NLB does not have security groups directly attached to it. Instead, the security groups of the target instances must be configured to allow traffic from the NLB. Thus, this option is incorrect.

Similar to option B, the NLB itself does not have a security group that must be attached to the target instance. Instead, it uses the security groups of the instances to control traffic. Therefore, this option is incorrect as well.

Network ACLs (Access Control Lists) on the subnet level must allow traffic from the NLB to the target instance. If the subnet’s network ACL is blocking the necessary traffic, health checks will fail, and the instance will not enter the InService state.

The target instance’s security group must allow traffic specifically from the NLB’s IP address range. If it is not configured to allow traffic from the NLB, the health checks will fail. This means that if the security group is not correctly set up to allow NLB traffic, it will cause health check failures.

Network ACLs are not attached to the NLB but rather to the subnets where the NLB and target instances reside. The configuration of the network ACLs impacts the instances directly, not the NLB itself. Therefore, this option is incorrect.