For option A order is fixed Any number of a and after that any number of b . so it cannot generate abbaa so wrong .
For option B equal number of a and equal number of b , bit order is not fixed but it can also not generate abbaa so wrong .
For Option C order is fixed either a or b or any number of a followed by any number of b , It also cannot generate abbaa so wrong.
For option D no order is fixed we can generate any string . so solution is D
ANSWER D