You have a set of applications, databases and web servers hosted in AWS. The web servers are placed behind an ELB. There are separate security groups for the application, database and web servers. The security groups have been defined accordingly. There is an issue with the communication between the application and database servers. In order to troubleshoot the issue between just the application and database server, what is the ideal set of MINIMAL steps you would take?

Answer options:

A.Check the Inbound security rules for the database security group. Check the Outbound security rules for the application security group.
B.Check the Outbound security rules for the database security group. Check the Inbound security rules for the application security group.
C.Check both the Inbound and Outbound security rules for the database security group. Check the Inbound security rules for the application security group.
D.Check both Outbound security rules for the database security group. Check both the Inbound and Outbound security rules for the application security group.

Answer – A
Here since the communication would be established inward to the database server and outward from the application server, you need to ensure that the Outbound rules for application server security groups and the Inbound rules for database server security groups are checked.
Option B is incorrect because it says that we need to check the outbound security group for the database, which is unnecessary. 
Option C is incorrect because you do not need to check for the Outbound security rules for the database security group.
Option D is incorrect because you do not need to check for Inbound security rules for the application security group.
For more information on Security Groups, please refer to the below URL:
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_SecurityGroups.html