In C++ program structure, what is required at the beginning of every program?

A main function is the starting point of every C++ program. It contains the code that is executed when the program is run. Option A is incorrect because the "using namespace std;" statement is not required in every program and can be omitted if the std namespace is explicitly used in the program. Option C is incorrect because the inclusion of the header is not required in every program and can be omitted if the program does not use any input/output operations. Option D is incorrect because not all programs need to define classes. In fact, some programs may not use any classes at all.