Linearly ordered elements means that all elements are arranged or listed have a particular relation. These elements can also be called as Partially ordered set(POSET). Ex: $a\leq b\leq c$ or $a\geq b\geq c$.
ith smallest element can be found by sorting. Best case here is if the elements are in a relation $a\leq b\leq c$ i.e when all elements are sorted in asc order. Retrieving ith smallest element would take constant time. But here we need worst case. Worst case scenario would be when elements have are in $a\geq b\geq c$ i.e when elements are in desc order. So we need to sort the elements in asc order and then find ith element. Best sorting algorithms take O(n log n) time.
So worst case time complexity for this problem would be, option C : O(n log n)