An element (either a neutral atom or an ion) can be distinguished from any other element by 1. The number of electrons it has. 2. The number of protons it has. 3. The number of neutrons it has. 4. The combined number of protons and neutrons it has

2. The number of protons it has

Explanation:

1. The number of electrons

This is incorrect because atoms and ions can have the same number of electrons from electron gain or loss

2. The number of protons it has

All atoms of an element have the same number of protons and every element has a different number of protons in its atoms. This is the correct option.

3. The number of neutrons it has

Examples includes carbon-12, nitrogen-13 and oxygen-14. They all have the same number of neutrons. This option is wrong

4. The combined number of protons and neutrons it has

Isobars are atoms that have the same mass number (proton and neutron). Examples are S-40, Cl-40, Ar-40 etc This option is wrong