A rational number is a number that has an ending to it. A irrational number is a number that continues on.
1) rt(64) is a rational number, because it breaks down into 8 x 8 , which can be moved outside the root (if there is a pair of numbers, it is moved out of the root and considered a whole number)
2) rt(84) is considered a irrational number, because it equals 9.16515138991...
3) B and D, for rt(25) = 5, and 1.15 have an end to the number, and does not continue forever
hope this helps