16) D

Possible digits that could make up the codes: 1, 2, 3, 4, 5, 6, 7, 8, 9

The best way to solve this is by trying out the digits given in the options given.

Is there a way to obtain 19 by not using 1 as any of the eight digits? No. Try using any of the possibilities and you will realise that 1 is needed in order to get 19.

Is there a way to obtain 19 by not using 3 as any of the eight digits? No. Try using any of the possibilities and you will realise that 3 is needed in order to get 19. 

Is there a way to obtain 19 by not using 5 as any of the eight digits? No. Try using any of the possibilities and you will realise that 5 is needed in order to get 19.

Is there a way to obtain 19 by not using 7 as any of the eight digits? Yes.

The combination could be:

9 + 6 + 3 + 1 = 19

8 + 5 + 4 + 2 = 19

Is there a way to obtain 19 by not using 9 as any of the eight digits? No. Try using any of the possibilities and you will realise that 9 is needed in order to get 19.