Explanation:

- When we talk about the solubility of salts, especially common ones like chlorides, nitrates, or sulfates, we usually refer to “Group 1” elements (alkali metals).

- Among the given options: Potassium, Rubidium, Sodium, and Lithium—all are alkali metals.

- Generally, salts of sodium (Na?), potassium (K?), and rubidium (Rb?) are highly soluble in water.

- However, as you go down the group, the lattice energy decreases faster than the hydration energy, making the salts of Potassium and Rubidium *even more soluble* than Sodium salts.

- Lithium salts are *less* soluble compared to others because of the small size and higher polarization.

- Correct Answer: Option 2, Rubidium — Rubidium salts are considered most soluble.

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