Your company is operating a traffic monitoring system which is based on thousands of different sensors, most of them being older, legacy devices and the rest are state of the art smart devices. As part of a transitioning project, you need to upgrade the solution by migrating it to Azure’s IoT platform. You want to make use of the no-touch device provisioning capabilities of DPS and you are planning the device registration process. 
Which attestation methods are applicable for the legacy and the new devices, respectively?

Answer options:

A.Symmetric key; HSM
B.Symmetric key; X.509
C.X.509; Symmetric key
D.X.509; TPM

Correct Answer: B
Option A is incorrect because the HSM (Hardware Security Module) is not an attestation method - it is a hardware-based solution for storing secrets on devices.
Option B is CORRECT because in most of the cases, symmetric key attestation is the only viable solution for low-resource legacy devices. More sophisticated smart devices, however, should use more secure ways of attestation, like X.509 certificates or Trusted Platform Modules.
Option C is incorrect because legacy devices are typically not able to accommodate and use certificates. In addition, for the smart devices having the capability to use more secure attestation mechanisms, using the simple symmetric key method is not recommended.
Option D is incorrect because, for the lack of resources or due to older technology, legacy devices are typically not able to accommodate and use certificates, hence X.509 attestation is not a right choice. For smart devices, TPM can be a viable option.
References: 

https://docs.microsoft.com/en-us/azure/iot-dps/how-to-legacy-device-symm-key
https://docs.microsoft.com/en-us/azure/iot-dps/concepts-service#attestation-mechanism