Comprehensive and Detailed in Depth Explanation:

The statement isTrue. Vault operates on a default-deny model for policies. The HashiCorp Vault documentation states: "Vault policies implicitly deny all actions that are not explicitly permitted in the Vault policy." This ensures that access must be explicitly granted, enhancing security.

The docs elaborate: "By default, a token has no policies attached beyond the default policy (which grants minimal permissions), and any action not explicitly allowed by an attached policy is denied." This principle underpins Vault’s access control, making A correct.