1. To solve this problem, you must keep on mind the followinwg information:
 You know that, by definition, an irrational number is a number that cannot be expressed as a fraction.
 2. Therefore, the correct answer is the option D. 
 3. To verify the answer you have:
 ∛16=2.51
 As you can see, the result is 2.51 and the decimal number 2.51 cannot be expressed as a fraction.