The correct answer is: A. SimpleDB
SimpleDB is a fully managed NoSQL database service that provides fast and easy access to structured data. It is a good choice for applications that need to store and retrieve large amounts of data without having to manage a database schema.
SQL Server is a relational database management system (RDBMS) that runs on Microsoft Windows. It is a popular choice for enterprise applications that require a high level of performance and scalability.
Oracle is a relational database management system (RDBMS) that runs on a variety of platforms, including Windows, Linux, and Solaris. It is a popular choice for enterprise applications that require a high level of performance and scalability.
RDS is a fully managed database service that makes it easy to set up, operate, and scale a relational database in the cloud. It offers a variety of database engines, including Amazon Aurora, MySQL, PostgreSQL, and Oracle.
In conclusion, SimpleDB is the correct answer because it is a NoSQL database service that does not require a database schema.