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

We need to find which element remains liquid inside pure boiling water (at 100°C).

For an element to be liquid at 100°C, its melting point must be below 100°C and its boiling point must be above 100°C.

Ga (Gallium): Melting point = 29.8°C, Boiling point = 2204°C. At 100°C, Ga is liquid. ✓

Br (Bromine): Melting point = -7.2°C, Boiling point = 58.8°C. At 100°C, Br₂ would be a gas. ✗

Li (Lithium): Melting point = 180.5°C. At 100°C, Li is still a solid. ✗

Cs (Caesium): Melting point = 28.5°C, Boiling point = 671°C. At 100°C, Cs is liquid. However, Cs reacts violently with water, so it would not "remain" as a liquid inside boiling water.

The correct answer is Ga (Gallium).