Impementation of Boolean function using decoders
Since there are 4 input variables, we can have 16 input combinations possible. And each of these combinations are numbered, from 0 to 15. The function represented by 

F(A,B,C,D)=П(0,4,6,7,9,11,12,13,15)


means that the output should be logical 0 for all the combinations listed in the bracket (maxterms). If the input combination does not belong to this list, then the output is 1.

Using the 2-level arrangement of decoders you have obtained 16 output lines each corresponding to an input combination (1 out of the 16). One of these lines will be 1 according to the input. And for output to become 1 that input should not belong to the maxterm list. 

In other words, the final output should be 1 if all the lines in the maxterm list are 0's. So connecting the lines corresponding to the maxterms (0,4,6,7,9,11,12,13,15) to a NOR gate will give you the output. A 9-input NOR gate is required for this purpose.