You need to ensure that when Resource Manager templates are deployed, the Resource Manager leaves unchanged resources that exist in the resource group that aren’t specified in the template. Which of the following mode in the template should be used to accomplish this requirement?

Answer options:

A.Complete
B.Nested
C.Incremental
D.Linked

Answer – C 
You need to specify the Incremental mode. 
This is also given in the Microsoft documentation. 

Since this is clearly given in the Microsoft documentation, all other options are incorrect. 
For more information on template deployment modes, please visit the below URL- 

https://docs.microsoft.com/en-us/azure/azure-resource-manager/deployment-modes