theta(nlogn) means our value can be >= as well <= of nlogn
T(n) is upper as well lower bounded by nlogn.
(A) is O(n2) so value of T(n) is less than n2 which satisfy lower bounded by nlogn
(C) is omega(n2) means T(n)>= n2 which is not possible because it is upper bounded by nlogn
(D) tells that it is less than <=nlogn which is fine