To quickly identify the NSG rule that is blocking the connection between VM2 and VM1, the Azure Network Watcher feature you should use is: A. Effective security rules The "Effective security rules" feature in Azure Network Watcher allows you to view the effective security rules applied to a network interface or a subnet. This feature is particularly useful in scenarios where you suspect an NSG rule is blocking connectivity, as it provides a comprehensive view of all the effective security rules, including those inherited from associated NSGs. By using the "Effective security rules" feature, you can quickly identify the specific rule that is blocking the connection between VM2 and VM1, without having to manually review the 100 inbound security rules in NSG1 and the 200 inbound security rules in NSG2. The other options are not as suitable for this scenario:

B. NSG flow logs: This feature is used for capturing network traffic flow information, which is not directly relevant to identifying the specific rule blocking the connection.
C. NSG diagnostic: This feature provides diagnostic information about NSGs, but does not directly help in identifying the specific rule causing the issue.
D. Connection troubleshoot: This feature is used for troubleshooting end-to-end connectivity issues, but does not focus on identifying the specific NSG rule causing the problem.

Therefore, the "Effective security rules" feature in Azure Network Watcher is the most appropriate choice to quickly identify the NSG rule that is blocking the connection between VM2 and VM1...