Mg
Reason — Alkaline earth metals are the elements in Group 2 of the periodic table. These include Be, Mg, Ca, Sr, Ba, Ra.
| Element | Group | Period | Type |
|---|---|---|---|
| Mg | 2 | 3 | Alkaline earth metal |
| Ca | 2 | 4 | Alkali earth metal, but present in period 4 |
| Na | 1 | 3 | Alkali metal |
| K | 1 | 4 | Alkali metal |
So, the alkaline earth metal of period 3 is Mg ( Magnesium )