Which of the following atoms has the greatest nuclear charge?

1. Al

2. Ar

3. Si

4. Na

Nuclear charge is the number of protons in the nucleus.

Nuclear charge = atomic number of the atom.

The atomic numbers of your atoms are:

Al - 13

Ar - 18

Si - 14

Na - 11

Thus Ar has the greatest nuclear charge.

so your answer is 2. Ar