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.