How can the user configure access from both the EC2 instances to RDS?

A user has launched one EC2 instance in the US East region and one in the US West region. The user has launched an RDS instance in the US East region.

How can the user configure access from both the EC2 instances to RDS?
A . It is not possible to access RDS of the US East region from the US West region
B . Configure the US West region’s security group to allow a request from the US East region’s instance and configure the RDS security group’s ingress rule for the US East EC2 group
C . Configure the security group of the US East region to allow traffic from the US West region’s instance and configure the RDS security group’s ingress rule for the US East EC2 group
D . Configure the security group of both instances in the ingress rule of the RDS security group

Answer: C

Explanation:

The user cannot authorize an Amazon EC2 security group if it is in a different AWS Region than the RDS DB instance. The user can authorize an IP range or specify an Amazon EC2 security group in the same region that refers to an IP address in another region. In this case allow IP of US West inside US East’s security group and open the RDS security group for US East region.

Reference: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithSecurityGroups.html