The answer is (3) 3. The outer shell of an atom is called valence shell. And the ground state is the lowest energy state of an atom. Aluminum has a electron distribution of 2, 8, 3. So the outer shell has 3 electrons.
If you look at aluminum (Al) in the Periodic Table of the Elements, you will see that its electron configuration is 2-8-3. The last number indicates the valence shell, which contains three electrons.
