After solving with K- Map , we get expression as (A+C) . (A+D) . (B+D)
which can be written as (A+ CD). (B+D)
= $ \overline{ \overline{(A+ CD).(B+D)} } $
= $ \overline{\overline{A+CD}+\overline{B+D}} $
= $ \overline{\overline{A+\overline{C'+D'}}+\overline{B+D}} $
As, variable are available both primed and Unprimed form. We are having 4 , 2-Input Nor Gates..