Final answer:
Strontium (Sr) is the alkaline earth metal among the given choices, as it is part of group 2 on the periodic table.
Step-by-step explanation:
The question regarding which element is an alkaline earth metal out of the choices Rb, Sr, Zr, or none can be answered by referencing the periodic table. Alkaline earth metals are located in group 2 of the periodic table and include the elements beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These are reactive, divalent metals that tend to form basic oxides upon reaction. Among the options provided, strontium (Sr) is the element that belongs to group 2 and is thus an alkaline earth metal.