Detailed Explanation:

Rationale for Correct Answer: terraform init installs and updates modules used by a Terraform configuration. After changing a module version constraint or version number, you must run terraform init so Terraform downloads the selected module version.

Analysis of Incorrect Options (Distractors):

B. terraform modules. Incorrect. There is no standard Terraform CLI command named terraform modules.

C. terraform plan. Incorrect. plan creates an execution plan but does not install updated module packages.

D. terraform apply. Incorrect. apply applies planned infrastructure changes, but module installation must be handled during initialization first.

Key Concept: Terraform modules are installed and updated during initialization.