Which command is used to manually mark a resource as tainted in Terraform?

The command used to manually mark a resource as tainted in Terraform is articulated through the specific syntax of 'terraform taint [address]'. In this context, 'tainting' a resource signals to Terraform that this resource is in a faulty state, and it should be destroyed and recreated on the next apply cycle. By using this command, you instruct Terraform to identify the designated resource (in this case, 'nullresource.backupdatabase') for this action.

This command is fundamental for scenarios where resources may not be behaving as expected, allowing users to ensure that changes take effect without needing to modify the actual configuration file. Marking a resource as tainted is discrete and specifically targeted, thereby enabling a flexible approach to resource management within the Terraform workflow.

The other commands provided represent actions that are either incorrect or do not match the required functionality. Marking a resource cannot be done through a 'mark' command or through an application trigger like 'apply' with an additional flag. Additionally, destroying the resource does not indicate that you wish Terraform to re-evaluate the resource; it simply instructs Terraform to remove it entirely without the context of recreating it.