You want to send cloud to device messages to your field devices. In order to minimize the risk of lost messages, you set the Max delivery count on the IoT Hub to 10. 
What happens to a message when its delivery fails 10 times?

Answer options:

A.Its status is set to Invisible
B.Its status is set to Expired
C.Its status is set to Dead lettered
D.Its status set to Enqueued

Correct Answer: C
Option A is incorrect because the message is in Invisible state while it is available for the target device for retrieval.
Option B is incorrect because the message becomes Expired when its time to live (ttl) time passes.
Option C is CORRECT because the IoT Hub attempts to deliver the messages max delivery count times After that number of failures, the IoT hub sets the state of the message to Dead lettered.
Option D is incorrect because Enqueued is the state where the message is put when the IoT Hub tries to deliver it. It means that the message is in a queue waiting to be retrieved by the target device.
Diagram: 

References: 

https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-messages-c2d#the-cloud-to-device-message-life-cycle
https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-endpoints