The series is having sum of positions of letters in each set as 41,51,41,51. Hence next two elements must have 41,51 as their sum of positions of letters . Hence FUN and SOAP is matching to the series .
Explanation: FUN's position is 6 (F) +21 (U) +14 (N) =41 .Same way for POT it is 51 , for BOX it is 41 and for SILK it is 51 .