A) f(n)=Ω(nlogn) means f(n)>=c* nlogn

D) f(n)=O(nlogn) means f(n)<=c* nlogn which means that no comparison based algo takes more than nlogn time. But we know there are algo like bubble sort, worst case of quick sort etc that takes n^{2} time which is more than nlogn. That is why option A.