The correct option is A) The program should contain at least one function.
Explanation: At least a C program lacking other user-defined functions should have main(). That is where a C program begins to run. Thus, although any user defined function is present or not in a given program, there must surely exist at least one, a main() function to carry on the execution process.
Why the others are incorrect:
B) There must not be any need for the functions in a program. This is simply incorrect as the main() function should always be added in any program.
C) Input data: Not all programs require input data. There are programmes that work without the interference of some inputs.
D) Output data: Such data is optional. Some applications do not display anything at all.
E) None of these are This is not true since the correct Answer is A