While operating an IoT infrastructure of a number of field devices, you need to select the right ways of how you communicate with the devices in certain practical scenarios. You are now writing the instructions for your development team about the cases when direct methods should be used. Direct methods are the right solution if you want ... 
Which are the two cases you would highlight in your instructions?

Answer options:

A.… to use HTTPS communication
B.… to restart multiple remote devices
C.… to send a simple notification to a specific device
D.… need immediate responses from your devices
E.… to set the device send telemetry frequency to 30 mins

Correct Answers: B and D
Option A is incorrect because in order to use direct methods, your devices must communicate either on AMQP or on MQTT protocols.
Option B is CORRECT because in order to launch commands on the devices, you need to invoke direct commands. Commands like restarting a device can be initiated via invoking direct methods.
Option C is incorrect because for one-way notifications from back-end services to specific devices the cloud-to-device messaging should be used.
Option D is CORRECT because direct methods are the only way of interactive, bi-directional communication with your devices. The device app can respond to the method immediately.
Option E 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. Direct methods are not the way of doing that.
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