It's gonna be Rb. The rule is that the lower it is and the closer it is to the left, the more reactive it will be.
Why left? Because their valence decreases, and the atom has a stronger desire to bond with other atoms. Alkali metals are considered the MOST reactive group in the periodic table, followed by AEM (alkaline earth metals). Noble gases are the least reactive because their valence's are full and they don't need 2 bond with anyone else. That's why artifacts are placed in noble-gas rich environments; so that they don't ruin and reactions don't occur (the declaration of independence is sealed in a helium container for that purpose)
Why down? Because there are more electron shells, resulting in a greater distance between the valence electrons and the nucleus. This reduces the attraction.
good luckkk daddyyyyyyy