You are building an industrial IoT solution for a manufacturing plant equipped with a number of “legacy” field devices which can’t use any of the protocols supported by the IoT hub, therefore they can’t be connected directly to the IoT hub and the end-devices don’t need to be registered in the IoT hub. Azure Edge provides the necessary functionality you can use to build the appropriate solution. 
Which type of solution should you use?

Answer options:

A.Identity translation gateway
B.Protocol translation gateway
C.Transparent gateway
D.Both A and B

Correct Answer: B
Option A is incorrect because while a Translation gateway must be used, it is clearly stated that the devices don’t need to be registered to the IoT hub, Identity translation is not the right option.
Option B is CORRECT because the Protocol Translation gateway is the only solution which can communicate with the downstream devices using industrial protocols and can forward their telemetry data to the IoT hub through a single channel, without requiring each device to have an IoT hub identity.
Option C is incorrect because Transparent gateway can be used in cases where the devices can connect via either MQTT or AMQP protocols. The gateway is “transparent” because it simply passes communications between the devices and IoT Hub.
Option D is incorrect because only the Protocol translation is suitable for this particular use case.
Reference: 

https://docs.microsoft.com/en-us/azure/iot-edge/iot-edge-as-gateway?view=iotedge-2018-06#translation-gateways