First off chlorine is not a metal so you can ignore that one.
Sodium and Rubidium are in group 1 of the periodic table and Magnesium is in group 2.
Group one metals are more reactive than group two because it is harder for the group two metals to lose their 2 valence (outer most) electrons.
As you go down group 1 there is an increase in the reactivity this is because as you go down there is an increase in the atomic radius which leads to more shielding. This weakens the electrostatic forces of attraction making it easier to lose the outermost electrons, therefore they are more reactive.
Explanation:
The answer would be B.
As paramagnetic with 3 unpaired electrons. Since there are 6 ligands around the Co+2 ion it isoctahedral and these ligands are neutral. This makes the overall charge on the complex +2 and therefore comes from the configuration for Co+2 which is [Ar] 3d7. Since it is in high spin you must fill all the orbitals with at least one electron and then pair up any that remain. If you do this, 3 unpaired electrons remain. Para magnetism occurs in substances with unpaired electrons.
