The answer is option a because it is the only option which best fits.
Using the law of exponents, whenever there is a fraction you would subtract the exponents.
Since there is not an exponent next to the numerator, there is nothing to subtract, thus leaving the answer A.
Additionally,
The answer would not be choice b because there is not a reason why 9 would be negative, the law of exponents does not state anything to change the number to a negative
The answer would not be option c because there is no need for the exponent to be 1/9 since it could not be, if we subtract or add exponents, there is nothing to subtract or add thus we would only end up with 9 and not 1/9.
Similar to the explanation above, nothing would change even if a negative sign is added, the above principals and explanations still apply to this choice.