Final answer:
As we move down a group in the periodic table, both atomic and ionic radii increase, while for an isoelectronic series, cations are smaller than anions.
Step-by-step explanation:
As we move down a group in the periodic table, both atomic and ionic radii tend to increase. This is because a new principal energy level (n) is added to the electron configuration, increasing the distance of the outermost electrons from the nucleus and thus the size of the atom or ion. When comparing ionic radii between groups, one must consider that such comparisons are valid only if the species being compared are isoelectronic, meaning they have the same number of electrons.
For an isoelectronic series of ions, cations are smaller than anions. This occurs because cations have fewer electrons compared to their parent atoms, resulting in less electron-electron repulsion and a stronger effective nuclear charge drawing the remaining electrons closer to the nucleus. In contrast, anions have more electrons, which leads to greater electron-electron repulsion and thus a larger radius. Therefore, the correct answer to the student's question is Option C) Increase, smaller.