We need to remember that:
xyz = (x)(y)(z)
if x, y and z are perfect cubes, then xyz would also be a perfect cube
Also,
(x^n)^m = x^(mn)
if it is a perfect cube, then it can be factored into the expression (x^n)^3, wherein n is a whole number.
All should be a perfect cube, given
215x^18(y^3)(z^21)
By writing the terms separately, we would see which one is to be changed.
215 = 5*43, we see this is not a perfect cube, we could changed it to 6^3 or 216
x^18=(x^6)^3
y^3=y^3
z^21=(z^7)^3
The other terms are perfect cubes except for the first one. So the correct answer would be the last option, 215 should be changed.