Since ionization energy decreases down the group and and increases across the period, the order from lowest to highest ionization energy is; Rb, Sr, Ca, Mg, Be.
In the periodic table, certain trends vary across the period and down the group. Trends like ionization energy increase across the period but decreases down the group.
Looking at the elements mentioned, the correct arrangement in order of the lowest to highest ionization energy is; Rb, Sr, Ca, Mg, Be.
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