Atomic Models

As we move across a period in the periodic table from left to right, the atomic radius decreases due to an increase in effective nuclear charge. Among the elements B, C, N, O, and F, fluorine (F) has the smallest atomic radius because it is furthest to the right in the second period.
Thus, the correct answer is (E).

For tripositive ions of lanthanides, the ionic radius decreases with increasing atomic number because the electrons are being removed from the 4f orbitals, which have poor shielding. Gd , being closer to the middle of the lanthanide series, has the smallest size.
Thus, the correct answer is (E).