There are 3 VPC`s that need instances to be able to pass traffic between each other. How can this be achieved.

Answer options:

A.Peer the VPC`s , transitive peering is now allowed in AWS
B.Peer the VPC`s and then Contact AWS support to enable transitive peering
C.Peer the VPC`s to each other in a full mesh configuration.
D.Peer the VPC`s , enable transitive peering via the route tables

Answer - C
This is clearly given in the AWS documentation
For more information on VPC Peering please refer to the below URL:
http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-peering.html