* Protons define an element: The number of protons in an atom's nucleus determines what element it is.

* Atomic Number: The atomic number of an element is the number of protons in its nucleus.

Here are the atomic numbers of the elements you listed:

* Carbon (C): 6 protons

* Oxygen (O): 8 protons

* Sodium (Na): 11 protons

* Aluminum (Al): 13 protons

Therefore, Aluminum (Al) has the most protons.