Which of the following would you expect to have properties most similar to Na, and why? K Mg N O Ar

Of the elements K (potassium), Mg (magnesium) N (nitrogen), O (oxygen) and Ar  (argon), K has the properties most similar to Na (sodium).

Elements that are in the same chemical family, as sodium and potassium are, have the most similar chemical properties. This is because they have similar electron configurations. Sodium, potassium and all other elements in Group 1A (alkali metals family) have one valence (outer) electron. They lose this electron very easily, forming a +1 ion. This gives results in the same stable electron configuration as a noble gas. The alkali metals are the most reactive of the metals because of their strong tendency to lose an electron. 

Since the alkali metals all form +1 ions they're found in similar compounds, such as NaCl and KCl, Na2O and K2O, and NaNO3 and KNO3. The all react with water to form hydrogen gas and OH-.

The other elements in the question are less similar because they're in different chemical families so their electron configurations are different. Mg is in Group 2A and has one valence electron. It forms a +2 ion. Nitrogen is in Group 5A and has five valence electrons. It forms a -3 ion. Argon is in Group 8A, the noble gases. It has a complete octet of 8 outer electrons so it doesn't gain or lose electrons, making it unreactive.