In general, the property (magnitudes only) that shows an opposite trend in comparison to other properties across a period is:

Across a period in the periodic table (for example from lithium to neon in the second period) the nuclear charge increases because one proton is added to the nucleus for every step to the right. At the same time, electrons are being added to the same principal energy level, so the extra nuclear charge is not completely shielded. Because of this, the effective nuclear charge, usually written as $$Z_{\text{eff}}$$, steadily rises from left to right.

Now, let us recall how the principal periodic properties depend on $$Z_{\text{eff}}$$:

1. Ionization enthalpy (also called first ionization energy) is the energy required to remove the outer-most electron. The greater the attraction between the nucleus and that outer electron, the higher this energy will be. Therefore, as $$Z_{\text{eff}}$$ increases across a period, ionization enthalpy also increases.

2. Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons. A higher effective nuclear charge pulls electrons more strongly, so electronegativity also increases from left to right in a period.

3. Electron gain enthalpy (or electron affinity) is the enthalpy change when an isolated neutral atom gains an electron to form an anion. A higher $$Z_{\text{eff}}$$ means the added electron is more strongly attracted, releasing more energy (the value becomes more negative). In magnitude, electron gain enthalpy therefore also increases across a period.

4. Atomic radius is half the distance between the nuclei of two identical bonded atoms. A rise in $$Z_{\text{eff}}$$ pulls the electron cloud closer to the nucleus. So, instead of increasing, the atomic radius actually decreases from left to right in a period.

We observe that ionization enthalpy, electronegativity and electron gain enthalpy all show an increasing trend across a period, while atomic radius shows a decreasing trend—exactly the opposite direction. Therefore, the property whose magnitude varies oppositely compared with the others is the atomic radius.

Hence, the correct answer is Option D.