The answer is A
Try breaking down each solution into cubic form.
A) a^8 = ((a^2)^3) (perfect cube because 2 is a prime number)
B) a^24 = (((a^2)^3)^2) (not a cube because of the 2)
C) a^28 = ((a^2)^2)^7 (not a cube because of 7)
B) a^64 = (((a^2)^32) (not a cube since 32 is not a factor of 3)
Hope I helped :)