Answer : The correct option is, (a) rubidium (Rb)
Explanation :
The general trend of the reactivity of the metal is,
The reactivity of the metals increases as we go down in the periodic table and the reactivity of the metals decreases as we go from left to right in the periodic table.
Therefore, the reactivity of metal in decreasing order will be,
Rb > Mg > Co > Cd > Ar
As per question, when the given elements dropped in water then the rubidium (Rb) is element which is most reactive that reacts rapidly with the water.
Hence, the element reacts rapidly with water is, rubidium (Rb).