Which method allows you to access multiple databases in PHP?

The reason that PDO (PHP Data Objects) is the correct answer is that it provides a unified interface for accessing different types of databases in PHP. PDO allows developers to interact with multiple database management systems (DBMS) using the same functions, which increases flexibility.

With PDO, you can establish a connection to various databases, such as MySQL, PostgreSQL, SQLite, and more, simply by changing the Data Source Name (DSN). It also supports prepared statements, which help prevent SQL injection attacks and enhance security. This abstraction layer enables a more modular approach when dealing with different database systems, making it an ideal choice for applications that might require connecting to multiple databases.

Other options like mysqli_connect(), while they do allow connection to a MySQL database, are limited only to that specific DBMS. mysqli_query() is a function used to execute queries on an already established MySQL connection, and db_access() is not a standard function in PHP for database connectivity. Therefore, only PDO provides the needed versatility for accessing multiple databases.