You are the global administrator of a company with a Microsoft 365 Subscription, and your users are on Win 10 devices. Some of your users occasionally visit a remote branch that provides limited or no cell phone coverage. Which authentication method should you configure for these users? 

Answer options:

A.FIDO2 security key
B.Voice
C.Email
D.SMS

Correct Answer: A 
FIDO2 security keys are typically stored on USB devices, and therefore not depended on cell phone coverage. 
FIDO2 security keys are an unphishable standards-based passwordless authentication method. Option B is incorrect. This authentication method needs cell phone coverage to work.
Option C is incorrect. This is not a valid authentication method for sign-ins. It can be used for password reset authentication only.
Option D is incorrect. This authentication method needs cell phone coverage to work.
Reference:
To know more about FIDO2 security keys, please refer to the link below: 

https://docs.microsoft.com/en-us/azure/active-directory/authentication/concept-authentication-passwordless#fido2-security-keys