Which of the following atoms or ions is the smallest? li , f- , na+, n , f, mg, al3+

Atomic Radii increases down the periodic table and to the left

So not looking at the ions largest to smallest goes

Li, Mg, N, F

Now just the ions

All the ions are the same because F- has 9 electrons + 1 = 10 electrons so Neon
Na + is 11 electrons - 1 = 10 electrons so Neon
Al 3+ is 13 electrons -3 = 10 electrons so Neon

Now with the Atoms and Ions together it would be 
LargestSmallest

Li, Mg, N, Ne, F

F atom is the smallest