While operating an IoT infrastructure with a number of field devices, from time to time, you have to ... … restart a device
… set the device send telemetry frequency to 5mins
… send a one-way notification to a specific device
… get immediate response from the device to
Which communication methods should you choose for the above tasks?

Answer options:

A.Use Cloud-to-device messaging; Set a desired property in device twin; Invoke a direct method; Invoke a direct method
B.Use Cloud-to-device messaging; Set a desired property in device twin; Use Cloud-to-device messaging; Invoke a direct method
C.Invoke a direct method; Set a desired property in device twin; Use Cloud-to-device messaging; Invoke a direct method
D.Invoke a direct method; Use Cloud-to-device messaging; Set a desired property in device twin; Invoke a direct method

Correct Answer: C
Option A is incorrect because commands like restarting a device can be initiated via invoking direct methods of the device app. C2D messaging is not suitable for this purpose.
Option B is incorrect because commands like restarting a device can be initiated via invoking direct methods of the device app. C2D messaging is for sending simple, one-way notifications from back-end services to specific devices.
Option C is CORRECT because in order to launch commands on the device you need to invoke direct commands; target values can be set via properties/desired in device twin; use C2D messaging for one-way notifications for the device; for two-way communication use direct methods.
Option D is incorrect because if you want to set target values for devices, you need to do it via setting the desired properties of the device twin.
References: 

https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-c2d-guidance
https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-messages-c2d