Answer:
The right answer is A, that is Rubidium (Rb)
Explanation:
Strontium has 2 electrons in the valance shell and is alkaline earth metal but relatively reactive.
Rubidium comes in period 5 and under alkali metals and very reactive towards oxygen and highly reactive than strontium and other option
This element has 2 electrons in its valence subshell (5s) according to Madelung rule.
Madelung rule that if the two orbitals have the same n+l value they fill in increasing order of n.
So the right answer is A, that is Rubidium (Rb)
