1. F = AB + C + D
2. F = (A'+B')C'D
3. F = A'B'+ C'+ D'
4. F = (A+B)CD
5. None of the above
   
   
   
   

1 is CORRECT. The front most NAND gate can be expressed by its equivalent symbol (an OR symbol with two bubbled inputs). Now the bubbles between the first two gates and third gate can be cancelled, resulting in a simplified circuit consisting of an AND and OR feeding an OR. The output expression is F = AB + C + D.

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