The page frame access sequence is
1,2,3.......19,20,21.....99,100,
1,2,3.......19,20,21.....99,100,
1,2,3.......19,20,21.....99,100,
1,2,3.......19,20,21.....99,100,
Using OPTIMAL replacement method from first 1....100 , first 20 page fault will take place as there are no common pages ,,from 21 to 100 again page fault will take place (these page fault will be equivalent to using MRU replacement method ). after these replacement the page frames will contain [1.....19,100] no pages .Considering the next sequence,no page fault from 1 to 19 and at the 20 page no ,19 will get replaced from the page frame and in the further sequence till 99 there will be page fault(these will be equivalent to using MRU ) and the resulting pages in frames will be 1...18 ,99,100 . going through the further sequence we will get the total page fault as 260 ......... replacing the pages using optimal method is equivalent to MRU replacement method thus answer (D)