Which one of the following elements will remain as liquid inside pure boiling water?

Gallium (Ga) remains as liquid inside boiling water, and the other elements mentioned in options can react with water.

Bromine is a halogen that reacts readily with water to form hydrobromic acid and hypobromous acid, which can further react to form bromic acid. Lithium and Caesium are alkali metals that react violently with water, producing hydrogen gas and the corresponding metal hydroxide.