It is d. the number of protons in the nucleus that makes an oxygen atom different from a carbon atom.
The atomic number Z, the number of protons in the nucleus, determines the identity of an element.
For carbon, Z = 6. For oxygen, O = 8.
This is how you decide which element is which.
The answer would be d) the number of protons.
The number of protons an element has in its nucleus is known as the atomic number and, like I've said, is unique to each atom.
Oxygen, on the other hand, is located in period 2, group 16 of the periodic table, which means it has and atomic number equal to 8 - 8 protons in its nucleus and 8 electrons surrounding the nucleus.
So, as a conclusion, you can never have two distinct elements with the same atomic number, i.e the same number of protons in the nucleus.