You have several IoT devices connected to your IoT Hub. You are designing the logic for your device applications that need to make certain operations on the corresponding device twins: Update reported properties
Replace tags
Retrieve tags
Observe desired properties
Replace desired properties
Retrieve device twin
Which of the above operations are available for device apps?

Answer options:

A.2, 3, 6
B.1, 3, 4, 6
C.1, 4, 6
D.2, 3, 5

Correct Answer: C
Option A is incorrect because the device app has no access to the tags section of the device twin. Neither for reading, nor writing.
Option B is incorrect because the device app has no access to the tags section of the device twin. Not even for querying.
Option C is CORRECT because the device app can retrieve the device twin document, can get notifications about the changing desired properties, and update the reported properties section in the device twin.
Option D is incorrect because these are all back-end operations available for services, not for device apps.
Reference: 

https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-device-twins