Here Working set window Size=5=Delta
The working-set window is a moving window.
At each memory reference, a new reference appears at one end and the oldest reference drops off the other end.
A page is in the working set if it is referenced anywhere in the working-set window. (Galvin;;Don't worry read the last paragraph to understand it simply)
Now As Told here penultimate reference (means except last reference) So take 5 page reference from end of the given list of references from the question except last reference. i.e.7 They are {3,3,9,6,2} in Working Set Window So the Working Set is {3,9,6,2}. So, option D is the Ans.
Note
Working Set Window::-- Examines the most recent Delta no. of page references.
Working Set::--Set of unique pages in most recent Delta page references.