An application consists of a front end hosted on EC2 instances in a public VPC subnet and RDS databases in a private VPC subnet.When attempting to establish a connection to the database, the application times out. What could be the source of this problem?

Answer options:

A.VPC Peering is not configured properly.
B.Database credentials are incorrect.
C.Database NSG is not configured to allow traffic from EC2 instances.
D.The public subnet does not have Internet Gateway configured.

Answer: C
Option A is incorrect because VPC peering is used to establish a secure connection between two VPC’s, not subnets.
Option B is incorrect because if database credentials were incorrect, the application would not time out. The database would return an error code immediately upon establishing a connection. The question specifically states that there is an issue establishing a connection.
Option C is CORRECT because VPC NSG’s on the RDS database subnet must be configured to allow inbound traffic from the public subnet on the database port.
Option D is incorrect because Internet Gateway is used in public subnets to route outbound traffic to the internet.
Reference:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Overview.RDSSecurityGroups.html