.
We want to have  x-y  maximum.
Under given condition, it happens when  x  is maximum possible and  y  is minimum possible.
According to this scheme,  x= 4, y = 1  is the  UNIQUE  solution.
ANSWER.  x= 4, y= 1  and the difference  x-y  is 3.
         Maximum  x-y  is  4-1 = 3.
         Option D.
Solved.