Your IoT solution consists of 10000 field devices, most of them being single field sensors, but also there are 50 edge devices used as translation gateways for a number of leaf devices. You need to implement a monitoring solution which can generate alerts on the basis of quota metrics, using the Azure Monitor. 
Which is the simplest way to configure the alert?

Answer options:

A.Use Resource logs with Total number of messages used
B.Use Platform metrics with Total number of messages used
C.Use Activity logs with Total device data used
D.Use Platform metrics with C2D message deliveries completed

Correct Answer: B
Option A is incorrect because in order to use resource logs for alerting or analysis, you need to route log data to a destination like Log Analytics or Storage Account. It requires additional configuration because you must create a diagnostic setting.
Option B is CORRECT because platform metrics are automatically collected by Azure Monitor and are available as time series data. Total number of messages can be used to monitor daily quota usage. You can easily set alerts from your IoT Hub / Alerts.
Option C is incorrect because activity logs are basically used to monitor the health of the services and administrative operations (to answer the who?, when?, what? questions) at a general level, not for monitoring service-specific metrics.
Option D is incorrect because in order to monitor daily quota usages, you need to use daily quota metrics like Total device data usage or Total number of messages used. C2D messages won’t fit your goal.
Diagram: 

References: 

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