Explanation:
To determine the smallest ion among the given options, we need to compare the atomic numbers and charges of each ion.

Atomic Numbers:
- Na (Sodium) has an atomic number of 11.
- Mg (Magnesium) has an atomic number of 12.
- K (Potassium) has an atomic number of 19.
- Al (Aluminum) has an atomic number of 13.

Charges:
- Na+ (Sodium ion) has a charge of +1.
- Mg2+ (Magnesium ion) has a charge of +2.
- K+ (Potassium ion) has a charge of +1.
- Al3+ (Aluminum ion) has a charge of +3.

Explanation:
The size of an ion is determined by the number of electrons and the effective nuclear charge. When an atom loses electrons to form a positively charged ion (cation), its effective nuclear charge increases, pulling the remaining electrons closer to the nucleus. This results in a smaller size compared to the neutral atom.

Among the given options, the Aluminum ion (Al3+) has the highest positive charge (+3) and the smallest atomic number (13). With a higher effective nuclear charge and fewer electrons, the Aluminum ion is the smallest ion.

It is important to note that the size of an ion is affected by other factors as well, such as the electron configuration and the presence of inner electron shells. However, in this case, we can determine the smallest ion based on the given information.

Therefore, the correct answer is option 'D' - Al3+ (Aluminum ion) is the smallest ion among the given options.