Your organization had setup a VPC with CIDR range 10.10.0.0/16. There are total 100 subnets within the VPC and are being actively used by multiple application teams. An application team who is using 50 EC2 instances in subnet 10.10.55.0/24 complains there are intermittent outgoing network connection failures for around 30 random EC2 instances in a given day. How would you troubleshoot issue with minimal configuration and minimal logs written?

Answer options:

A.Create a flow log for the VPC and filter the logs in CloudWatch log group.
B.Create flow log for each EC2 instance network interface one by one and troubleshoot the connection issue.
C.Create a flow log for subnet 10.10.55.0/24.
D.None of the above.

Answer: C
VPC Flow Logs captures IP traffic going to and from network interfaces in your VPC. Flow log data is stored using Amazon CloudWatch Logs. After you`ve created a flow log, you can view and retrieve its data in Amazon CloudWatch Logs.
You can create a flow log for a VPC, a subnet, or a network interface. 
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/flow-logs.html#flow-logs-basics
VPC Flow Logs capture following information and logs them to CloudWatch logs,
version account-id interface-id srcaddr dstaddr srcport dstport protocol packets bytes start end action log-status
Find more information about each record here. 
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/flow-logs.html#flow-log-records
For option A, although creating a flow log for entire VPC would work, it captures lot of unrequired information from rest 99 subnets and finding out the affected EC2 instances from CloudWatch logs would become really troublesome.
For Option B, creating flow log at each EC2 network interface would work, but it takes log of configuration and time consuming trial and error troubleshooting.
For Option C, creating a flow log for the subnet would capture just the traffic going in and out of the subnet. This would help us identify the network trace for the affected EC2 instances and find out the root cause in timely manner.