What does Terraform require for managing module dependencies effectively?

Terraform requires explicit module versioning for managing module dependencies effectively because it ensures that the correct and compatible versions of modules are used in your configurations. By specifying the version of each module, you can avoid issues that might arise from breaking changes introduced in newer versions, thereby maintaining stability in your infrastructure.

Explicit module versioning allows team members to understand which version of a module is being used, ensuring that the code behaves consistently across different environments and deployments. This practice also facilitates easier upgrades and changes, as the impacts of modifying a module version can be better assessed and managed.

The other options do not address the critical nature of versioning in dependency management. Proper documentation, while important, does not directly influence the operational aspects of how Terraform handles versions or dependencies. Relying only on variables would not ensure the integrity or compatibility of modules across changes or environments. Ignoring dependency management altogether leads to unpredictable infrastructure behavior and potential failures during deployments.