You are developing a .Net web application that stores data in Azure Cosmos DB. The application must use the Core API and allow millions of reads and writes. The...

C: The UseMultipleWriteLocations of the ConnectionPolicy class gets or sets the flag to enable writes on any locations (regions) for geo-replicated database accounts in the Azure Cosmos DB service.
Note: Once an account has been created with multiple write regions enabled, you must make two changes in your application to the ConnectionPolicy for the
Cosmos client to enable the multi-region writes in Azure Cosmos DB. Within the ConnectionPolicy, set UseMultipleWriteLocations to true and pass the name of the region where the application is deployed to ApplicationRegion. This will populate the PreferredLocations property based on the geo-proximity from location passed in. If a new region is later added to the account, the application does not have to be updated or redeployed, it will automatically detect the closer region and will auto-home on to it should a regional event occur.
Azure core API application " ConnectionPolicy class" cosmos db multiple write regions enabled
D: With multi-region writes, when multiple clients write to the same item, conflicts may occur. When a conflict occurs, you can resolve the conflict by using different conflict resolution policies.
Note: Conflict resolution policy can only be specified at container creation time and cannot be modified after container creation.
Reference:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.azure.documents.client.connectionpolicy
https://docs.microsoft.com/en-us/azure/cosmos-db/sql/how-to-multi-master
https://docs.microsoft.com/en-us/azure/cosmos-db/sql/how-to-manage-conflicts