You have two VPCs in different regions ( VPC A and VPC B) peered with each other. You have created an EFS for VPC A. When you tried to mount the EFS on EC2 instances on VPC B, you are getting a connection timed out error. What can cause this? (Select TWO)

Answer options:

A.AWS EFS takes upto an hour after creation to make mount targets available.
B.Security group is improperly configured for the EFS mount target.
C.Security group on mount targets does not have inbound NFS port open to VPC B’s EC2 instances.
D.EFS cannot be mounted through VPC peering.

Correct Answer: B and C
You can connect to Amazon EFS file systems from EC2 instances in other AWS regions using an inter-region VPC peering connection, and from on-premises servers using an AWS VPN connection. 
Option A is incorrect. Usually, EFS and its mount targets get created within a few moments.
Option B is CORRECT because both an Amazon EC2 instance and a mount target have associated security groups. These security groups can act as a virtual firewall that controls the traffic between them
Option C is CORRECT because the security groups you associate with a mount target must allow inbound access for the TCP protocol on the NFS port from all EC2 instances on which you want to mount the file system.
Option D is incorrect because VPC peering within a single AWS Region when using certain Amazon EC2 instance types is supported. Inter-region VPC peering is supported for all instance types
Reference:
https://docs.aws.amazon.com/efs/latest/ug/limits.html
https://aws.amazon.com/about-aws/whats-new/2018/10/amazon-efs-now-supports-aws-vpn-and-inter-region-vpc-peering/