The correct option is [resourceGroup().location].

[resourceGroup().location] returns the location property of the resource group that contains the deployment so it ensures each deployed resource can use the same region as the resource group. Using this function reads the actual resource group metadata and so it is the reliable way to get the deployment region.

[parameters('deployLocation')] is incorrect because that expression refers to a parameter value supplied to the template and it does not automatically reflect the resource group location unless you explicitly pass the group location into the parameter.

[environment()] is incorrect because the environment function provides information about the Azure environment and endpoints and it does not return the resource group region.

[subscription().location] is incorrect because the subscription object does not expose a location property for resource placement and subscription scope is not the right place to look for the resource group region.

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