Final answer:
Rubidium (Rb) has the lowest first ionization energy among the elements listed in the question.
Explanation:
Rubidium (Rb) will lose an electron most easily and has the lowest 1st ionization energy among the options provided (Ca, Sr, Rb, K). This is because as we move down a group in the periodic table, the ionization energy decreases due to the increasing atomic size and shielding effect of inner electrons.
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