Answer is (1) - atomic number.
Atomic number is unique for an element. The elements in periodic table are organized according to the atomic number of elements. Another thing is atomic number is equal to number of protons of the element.
Mass number can be defined as the sum of proton number and neutron number. Although proton number is same for the atoms of same element, the neutron number can be varied. The atoms which has same atomic number but have different number of neutrons are called isotopes. Hence, mass number not enough for the identification of elements.
Only the neutron number is also not enough for the identification of elements. But if you have both mass number and the neutron number, then you can find atomic number by using mass number = proton number + neutron number equation. Then you can identify the element.
If you have total number of electrons of the element when it is in its neutral state you can identify the element because at neutral state number of electrons = number of protons (atomic number). But if only the valence electrons are given then you cannot find the element exactly because usually all elements in certain group have same number of valence electrons.
