In most of its compounds, this element exists as a monatomic cation.

1. Br
2. N
3. Na
4. Cl
5. O

Is it 3. Na ?

yes.

Yes, the correct answer is 3. Na (sodium). Sodium is a highly reactive alkali metal that tends to lose one electron to form a monatomic cation with a charge of +1. In most of its compounds, sodium exists as Na+, which is a monatomic cation.

To determine the answer, you could look up the chemical properties of each element listed and their common oxidation states. In this case, sodium is known to have a tendency to lose one electron, while the other elements listed do not typically form monatomic cations in their compounds. Therefore, sodium (Na) is the element that most commonly exists as a monatomic cation.