Explanation:

- Most alkali metal salts are soluble in water, but there are slight differences.

- Lithium salts tend to be less soluble compared to the others.

- Potassium, rubidium, and sodium salts are generally highly soluble.

- However, as we go down the group, solubility increases.

- Rubidium is below potassium and sodium in the periodic table.

- This means Rubidium salts are generally the most soluble among these.

- Sodium salts are very soluble, but not as much as potassium or rubidium.

- Correct Answer: Option 2 - Rubidium