Hey could anyone assist me with this problem?

Find the least integer n such that f(x) is O(x^n) for each of these functions.

a) f(x) = 2x^2+x^3logx
b) f(x) = 3x^5+(logx)^4
c) f(x) = (x^4+x^2+1)/(x^4+1)
d) f(x) = (x^3+5logx)/(x^4+1)