Two Amazon EC2 Instances in Different Subnets Should Be Able

Two Amazon EC2 instances in different subnets should be able to connect to each other but cannot. It has been confirmed that other hosts in the same subnets are able to communicate successfully, and that security groups have valid ALLOW rules in place to permit this traffic. Which of the following troubleshooting steps should be performed?

A) Check inbound and outbound security groups, looking for DENY rules
B) Check inbound and outbound Network ACL rules, looking for DENY rules
C) Review the rejected packet reason codes in the VPC Flow Logs
D) Use AWS X-Ray to trace the end-to-end application flow