The least reactive element in group 1-A is .
About Group 1 (A) :
In the modern periodic table(or long form of periodic table), the group 1-A elements( alkali metals) are Lithium(Li) ,Sodium(Na), Potassium (K), Rubidium(Rb) and Cesium (Cs).
Reactivity Trend:
Hence, The least reactive element in group 1-A is Lithium(Li).