Rb
Explanation:
Rb is an element in Period 5 that is the most active metal from the given choices.
The first group on the periodic table are made of highly reactive metals due to their large electropositivity.
Electropositivity or metallicity is a measure of the tendency of atoms of an element to lose electrons.
As we go from left to right on the periodic table, electropositivity decreases.
Therefore, in period 5, we have elements: Rb Sr, Rb is more reactive and most active of the two.
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