Physical changes involve a change in the physical properties of a substance without altering its chemical composition. In this case, the physical change is represented by option D, which is adding NaCl to water. Let's discuss why this is a physical change in detail.

Explanation:
When NaCl (sodium chloride) is added to water, it dissolves and forms a homogeneous mixture. This process is called dissolution or dissolving. The NaCl molecules disperse evenly throughout the water, resulting in a clear and colorless solution. This is a physical change because the chemical composition of both NaCl and water remains the same before and after the addition.

Key points:
- Physical changes do not involve the formation of new substances.
- The physical state, shape, size, or phase of the substance may change in physical changes.
- In this case, the physical change occurs when NaCl dissolves in water to form a solution.

Other options:
Let's briefly discuss the other options and why they are not physical changes.

a) Burning magnesium in air:
Burning magnesium in air is a chemical change. Magnesium reacts with oxygen in the air to form magnesium oxide. The chemical composition of magnesium changes during this process, making it a chemical change.

b) Mixing NH3 and HCl:
Mixing ammonia (NH3) and hydrochloric acid (HCl) also results in a chemical change. Ammonia reacts with hydrochloric acid to form ammonium chloride. The chemical composition of the substances changes, indicating a chemical change.

c) Mixing BaSO4 and NaCl:
Mixing barium sulfate (BaSO4) and sodium chloride (NaCl) does not result in a reaction or any noticeable change. However, both substances retain their original chemical composition, making it a physical mixing or a physical change.

Conclusion:
Among the given options, adding NaCl to water is the only physical change. The other options involve chemical changes as they result in the formation of new substances with different chemical compositions.