To answer this question, we need to understand the concept of the return value of the main() function in a program.
In most programming languages, the main() function is the starting point of a program and is required to have a return type. The return type of the main() function indicates the status of the program execution.
Option A) 0 - This option is correct. In most programming languages, including C and C++, the main() function should return an integer value. By convention, returning 0 from the main() function indicates that the program executed successfully without any errors.
Option B) 1 - This option is incorrect. Returning 1 from the main() function usually indicates that the program encountered an error or some exceptional condition during execution.
Option C) void - This option is incorrect. The return type of the main() function cannot be void in most programming languages. The main() function must have a return type of int.
Option D) Don't Know - This option is incorrect. We do know the value that the function main() should return if the program completes executing successfully.
Therefore, the correct answer is option A) 0. This option is correct because returning 0 from the main() function indicates that the program executed successfully without any errors.