Which one of the following elements is least reactive with water ?

The reactivity of alkali metals (Group 1) with water increases as you move down the group in the periodic table. This trend occurs because the atomic radius increases and the first ionization energy decreases down the group, making it easier for the atom to lose its outermost electron. Among the options provided, Lithium (Li) is at the top of the group, followed by Sodium (Na), Potassium (K), and Cesium (Cs) further down. Consequently, Lithium is the least reactive of the four, reacting steadily and fizzing without the violent or explosive nature seen in its heavier counterparts. While Sodium and Potassium react vigorously or explosively with cold water, often catching fire, Lithium's reaction is relatively gentle and slow. Therefore, Lithium is the least reactive element with water in this series.