Comprehensive and Detailed In-Depth Explanation:

The storage backend is configured in the Vault configuration file. The Vault documentation states:

"The Vault configuration file includes different stanzas and parameters to define a variety of configuration options. These configurations include the storage backend, listener, TLS certificates, seal type, cluster name, log level, UI, cluster IP address, and a few more. Most of these are required to get Vault up and running in the first place, so they must be placed in the configuration file."

—Vault Configuration

C: Correct. For Integrated Storage:

"Configuring the storage backend to be used by Vault is done in the Vault configuration file."

—Vault Configuration: Raft Storage

A: systemd manages the service, not storage.

B: Backend must be set before running.

D: Agent sink is for client tokens.