The answer is Cs, no. 2.
Cesium (Cs) is an element which has an outermost electron with most energy in the ground state.
Cesium (Cs), Lithium (Li), Potassium (K), and Sodium (Na) are elements which belong to the group 1 family, the alkali metals. Each has a valance of 1 and wants to release/ lose this e- (electron) to be isoelectronic with the nearest noble gas. Based on the periodic trends, an atom’s radii will raise going down a group.