The lightest liquid metal is

At room temp liquid metal Cs, Hg
At high temp liquid metals Cs, Fr
The lightest metal is Cs

Among the metals, cesium (Cs) is the lightest liquid metal. It remains in liquid form at room temperature, with a relatively low melting point of 28.5°C. This is lower than many other metals, making it the lightest liquid metal.

• Mercury (Hg) is often known as the only metal that is liquid at room temperature, but it is denser than cesium.
• Gallium (Ga) is another low-melting-point metal, but its density is still higher than cesium.

Thus, the correct answer is C: Cs (Cesium), which is the lightest liquid metal.