The definition for Big O notation is -For Constants N and c ,if for some n>N ,f(n)<=c*g(n) then we can say that g(n) bounds f(n)
OR
f(n)=O(g(n))
Here f(n) is (log n)^c and g(n) is n.
so for any constant c>0,it can be easily proved that (log n)^c<=n.
Hence (log n)^c=O(n)