Explanation/Reference:
Database Schema Service uses schema isolation to implement multi-tenancy, which allows full transparency while still providing efficient use of database resources. The Oracle Database is, at its core, a multiuser system for sharing data, so Database Schema Service simply uses the capabilities built up for the Oracle Database to share resources among multiple Database Schema Service customers.
Incorrect Answers:
E: Oracle Database Cloud - Database Schema Service has four main components:
Oracle Database 11gR2 Enterprise Edition.

Oracle Application Express 5. Used to create and deploy all varieties of applications in a browser-

based environment.
RESTful Web Services. Allows access to the data in your Database Schema Service through simple

URIs.
Packaged Applications and Sample Code.

References: https://docs.oracle.com/cloud/latest/dbcs_schema/CSDBU/GUID-B1C86AD3-D36D-461F- B7B1-37C8EB05DBE8.htm