Introduction:
Cations are positively charged ions that are formed when an atom loses one or more electrons. The size of a cation depends on its atomic radius, the number of protons in its nucleus, and the number of electrons it has lost. In this context, we will discuss which of the given cations is the smallest and why.

Explanation:
The atomic radius of an element decreases as we move from left to right in a period of the periodic table. This is because the number of protons increases, which increases the positive charge on the nucleus, pulling the electrons closer to it. Therefore, the cation formed from an element in a period will be smaller than the cation formed from an element in the previous period.

1. Na+ (Sodium ion):
Sodium is present in the third period of the periodic table. When it loses one electron, it forms a cation with a 1+ charge. The atomic radius of sodium is 186 pm, and the radius of Na+ is 102 pm. Therefore, the ionic radius of Na+ is smaller than that of the parent atom.

2. Ca2+ (Calcium ion):
Calcium is present in the fourth period of the periodic table. When it loses two electrons, it forms a cation with a 2+ charge. The atomic radius of calcium is 197 pm, and the radius of Ca2+ is 100 pm. Therefore, the ionic radius of Ca2+ is smaller than that of the parent atom.

3. Mg2+ (Magnesium ion):
Magnesium is present in the third period of the periodic table. When it loses two electrons, it forms a cation with a 2+ charge. The atomic radius of magnesium is 160 pm, and the radius of Mg2+ is 72 pm. Therefore, the ionic radius of Mg2+ is smaller than that of the parent atom.

4. Al3+ (Aluminum ion):
Aluminum is present in the third period of the periodic table. When it loses three electrons, it forms a cation with a 3+ charge. The atomic radius of aluminum is 143 pm, and the radius of Al3+ is 53 pm. Therefore, the ionic radius of Al3+ is smaller than that of the parent atom.

Conclusion:
From the above discussion, it is clear that the smallest cation is Al3+ with an ionic radius of 53 pm. This is because aluminum has a smaller atomic radius than the other cations, and it loses three electrons to form the cation, which further reduces its size.