In a certain code language, 'this is fact' is written as 'cuz duz ruz', 'fact is fictional' is written as 'guz ruz cuz', and 'doubt is clear' is written as 'cuz kuz buz'. How will 'this is fictional' be written in that language?

duz guz cuz

Given; In a certain code language, 'this is fact' is written as 'cuz duz ruz', 'fact is fictional' is written as 'guz ruz cuz', and 'doubt is clear' is written as 'cuz kuz buz'. 

So,

this  is  fact  =  cuz  duz  ruz

fact  is  fictional  =  guz  ruz  cuz

doubt  is clear  =  cuz  kuz  buz

Thus,

this is fictional is coded as duz cuz guz.