If I need to answer I go with 1st and 3rd,merge sort is nlogn, in Big-oh it is T(n)<=cn^2,but merge sort never can be O(n^2)
and same with omega T(n)>=c.n,but merge always be greater than (n),it but in little-oh say T(n)<c.n^2 and little omega T(n)>c.n,