What command can be used to update a Vault policy named web-app-1 using the command...

Comprehensive and Detailed In-Depth Explanation:

To update an existing Vault policy via the CLI, the correct command is vault policy write:

D. vault policy write web-app-1 web.hcl: This command updates (or creates if it doesn’t exist) the policy named "web-app-1" with the contents of "web.hcl". The documentation states: "The write keyword is used to update an existing policy with the contents of the specified file."

Incorrect Options:

A. vault policy create: No such subcommand exists; create is invalid. "The create keyword is not a valid subcommand."

B. vault policy fmt: Formats the HCL file but doesn’t update Vault. "It is used to format a policy file."

C. vault policy update: Incorrect syntax; Vault uses write, not update. "There is no update command, only write."

The write command’s dual purpose (create or update) simplifies policy management.