Which is the base table of the configuration management database hierarchy?

`cmdb_rel_ci` is a table for storing relationships between CIs, not the base table of the CI hierarchy itself.

`ucmdb` is not a standard ServiceNow table; it might refer to a different CMDB product.

The `cmdb_ci` table is the base table for all Configuration Items (CIs) in the CMDB hierarchy, meaning all other CI classes extend from it and inherit its fields and behaviors. While `cmdb` is the top-level CMDB table, `cmdb_ci` is the direct parent for all actual Configuration Items.

The `cmdb` table is the base table for the entire CMDB application, but `cmdb_ci` is specifically the base for Configuration Items within that hierarchy, which is typically what 'CMDB hierarchy' refers to in this context.