That was simple, wasn't it! As you start filling an atom with electrons, you always start with the orbit closest to the nucleus. So, the answer is \(n=1\).
If you add 1 more electron to Hydrogen (and 1 proton and 2 neutrons to the nucleus, not shown in the animation), we get Helium. The added electron also goes to the \(n=1\) orbit.

If you now add 1 more electron to Helium (and 1 proton and 2 neutrons), you get Lithium. Remember that \(n=1\) can have a maximum of \(2n^2 = 2 \times 1^2 = 2\) electrons (that's why the green tick in the table). So, the 3rd electron has to go into the \(n=2\) orbit.