Explanation:

- Lithium:

- Found in Group 1 of the periodic table (alkali metals).

- This small-sized metal has the highest boiling point among alkali metals due to stronger metallic bonds.

- Sodium:

- Next in Group 1, larger atomic size than lithium.

- Lower boiling point than lithium due to weaker metallic bonds.

- Potassium:

- Larger than sodium and lithium, following a decrease in boiling point.

- Rubidium:

- Even larger atomic size, with the lowest boiling point among the four listed.

Answer: Lithium

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