The correct answer is A. header.
A header file is a file that contains declarations of functions, classes, and other data types. It is used to provide information to other files that need to use the declarations. Header files are typically stored in a .h file extension.
A program file is a file that contains the source code for a program. It is used to compile and run the program. Program files are typically stored in a .cpp file extension.
A source file is a file that contains the source code for a program or library. It is used to compile and link the program or library. Source files are typically stored in a .c or .cpp file extension.
A text file is a file that contains text. It can be used to store data, configuration information, or anything else that can be represented as text. Text files are typically stored in a .txt file extension.
In order to use a class to define objects in many different programs, you should define the class in a header file. This will allow other programs to include the header file and use the class.