Here it says which of the following atoms has the smallest ionization energy. Remember, the general trend is as we're heading towards the top right corner, your ionization energy should be increasing.
So if we take a look, we have phosphorus, we have fluorine, potassium, chromium and bromine. We want the smallest ionization energy. So we're looking for something that's closer to the left side and lower down for the periodic table.
Out of the choices presented, the one that fits that definition the best is potassium. It's the one most to the left and lowest down in terms of the periodic table, and therefore to have the smallest ionization energy.
