The ionic radii of elements exhibit the same trend as the atomic radii. The atomic size generally decreases across a period for the elements of the second period. It is because, within the period, the outer electrons are in the same valence shell and the effective nuclear charge increases as the atomic number increases resulting in the increased attraction of electrons to the nucleus. The size of an anion will be larger than that of the parent atom because the addition of one or more electrons would result in increased repulsion among the electrons and a decrease in effective nuclear charge. In given question option A and C are cations in the second period. So they will have less ionic radius than that of the anions o same period. oxygen falls left to the Fluorine and also has one extra negative charge (O2−2) than Fluorine ion (F−)