. . . . . . . . file very necessary to connect to the access data base file by ADO.

The correct answer is: A. .mdb

An .mdb file is a Microsoft Access database file. It is a binary file that stores data and objects used in an Access database. ADO (ActiveX Data Objects) is a programming interface that allows applications to access data from a variety of data sources, including Access databases. To connect to an Access database using ADO, you need to create an ADO connection object. The connection object specifies the location of the database file and the credentials that are required to access the database.

The other options are incorrect because they are not valid file extensions for Access database files.

• .cls is a class file.
• .frm is a form file.
• .acca is not a valid file extension.