1. (AB)' + (X'Y)
2. A'*B' + XY'
3. (A'+ B')(X + Y')
4. (AB + X'*Y)'
5. (AB)'(X'*Y)'

3 is CORRECT. DeMorgan's Theorem is applied 2 times:

F' = (AB + X'Y)' 
   = (AB)'(X'Y)'       First time: (x + y)' = x'*y'
                       (Let x = AB and y = X'Y)
   = (A'+ B')(X + Y')  Second time.  Use: (x*y)' = x' + y'

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