The answer should be 2) electrons.
The elements in the periodic table are arranged by increasing atomic number, which is also the number of protons in an element. For example, Carbon has 6 protons, it is the sixth element on the table. While Nitrogen, which has 7 protons, is after Carbon.
In atoms, the number of protons equals to the number of electrons too, if the number of electrons does not equal to the number of protons, the substance is no longer atom, but ion. So, the answer to this is 2) electrons, as the elements on the table is arranged according to increasing atomic numbers.
