Which of the following atoms has the lowest ionization potential?

Since the ${}_{55}{}^{133}\mathrm{Cs}$ has larger size among the four atoms gives, thus the electrons present in the outermost orbit will be away from the nucleus and the electrostatic force experienced by electrons due to nucleus will be minimum. Therefore the energy required to liberate electron from outer will be minimum in the case of ${}_{55}{}^{133}\mathrm{Cs}$.