Which of the following elements is the radioactive alkaline earth...

The alkaline earth metals belong to Group 2 of the periodic table, which includes Beryllium ($\text{Be}$), Magnesium ($\text{Mg}$), Calcium ($\text{Ca}$), Strontium ($\text{Sr}$), Barium ($\text{Ba}$), and Radium ($\text{Ra}$).

Among these elements, Radium ($\text{Ra}$) is the only naturally occurring radioactive alkaline earth metal.

Cesium ($\text{Cs}$), Rubidium ($\text{Rb}$), and Francium ($\text{Fr}$) are alkali metals (Group 1), with Francium being the radioactive alkali metal.

Answer: $\text{Ra}$