Which two things do we need to confirm in the VPC settings so that these EC2 instances can communicate inside the VPC?

Explanations:

Network ACLs (Access Control Lists) control inbound and outbound traffic at the subnet level. To ensure communication between the two EC2 instances in different subnets, the network ACLs must be configured to allow traffic between those subnets. If the ACLs block this traffic, the instances will not be able to communicate.

The instance class and key pair do not affect the ability of EC2 instances to communicate within a VPC. These factors are more related to instance performance and access management rather than network connectivity. Therefore, this option is irrelevant to ensuring communication between the two instances.

The default route to a NAT instance or Internet Gateway (IGW) is necessary for outbound internet traffic, but since both EC2 instances are within the same VPC and communicating internally, they do not require a NAT or IGW for communication. Thus, this option does not pertain to the internal communication between instances.

Security groups act as virtual firewalls for EC2 instances, controlling inbound and outbound traffic at the instance level. For the application instance to communicate with the database instance, the security group attached to the database instance must allow traffic from the application instance on the appropriate port and protocol. Without the correct security group settings, the instances will not be able to communicate.