Your IoT solution consists of 10000 field devices, most of them being single field sensors. In addition,there are 20 edge devices which serve as translation gateways for a number of leaf devices. You need to implement a monitoring solution which is able to provide (1) general information of the connected devices and (2) specific, device level information (connection status etc.) of the gateway devices as critical components. 
Which monitoring services are you recommended to use?

Answer options:

A.Use Azure Monitor for the general view; configure Event Grid for the gateways
B.Use Azure Monitor for both the sensors and the gateways
C.Use Event Grid for the sensors and the gateways
D.Event Grid for the field sensors; Azure Monitor for the gateways

Correct Answer: A
Option A is CORRECT because for the generic view of the connected devices, in most cases the Azure Monitor’s Connected devices metric is a good choice because it reports the number of devices connected to your IoT Hub. For the critical elements of the infrastructure, however, Event Grid’s per-device monitoring solution is recommended.
Option B is incorrect because Azure Monitor is great for “bulk” monitoring since it provides a metric Connected devices which, for example, can be used to trigger actions when it drops under a number. Per-device monitoring, however, is not supported by Azure Monitor.
Option C is incorrect because Azure Monitor’s general metrics are sufficient for the majority of the devices. You need Event Grid’s per-device capabilities for the gateways only.
Option D is incorrect because Azure Monitor doesn’t provide the device-level monitoring capabilities.
Reference: 

https://docs.microsoft.com/en-us/azure/iot-hub/monitor-iot-hub#monitor-per-device-disconnects-with-event-grid