You can see that the atomic number of 8 is found in the periodic table which is oxygen.
There are 4 atoms of oxygen in Cu(NO2)2 because the subindex of oxygen is 2 and this is inside of a parenthesis which show us another subindex telling us how many times NO2 is bonded to Cu, which in this case is twice. So, there are 2 x 2 = 4 oxygen atoms.
And for 5 moles of water, you can see that there are five molecules of water and each molecule contains 1 oxygen atom, so it would be 1 x 5 = 5 oxygen atoms.
The final step is sum the oxygen atoms which is 4 + 5 = 9 oxygen atoms.
The answer is C. 9.
