In which of the following services would you enable "just-in-time VM access "?

Answer options:

A.Azure Bastion
B.Azure Firewall
C.Azure Front Door
D.Azure Security Center

Answer : D 
You can enable this feature in Azure Security Center as shown below. You need to go to Azure Defender and then access the just in time feature. 

As this is clear from the implementation, all other options are incorrect 
For more information on Just in time VM access, please visit the below URL 

https://docs.microsoft.com/en-us/azure/security-center/security-center-just-in-time