Which one of the following metals is in liquid form at room temperature?

At room temperature, mercury (Hg, atomic number 80) is the only metal that remains in a liquid state.

The Science: Metals usually form strong metallic bonds where valence electrons are shared freely among a lattice of atoms, making them solid and rigid. However, mercury's electrons are bound unusually tightly to its nucleus (a phenomenon amplified by relativistic effects). Because its atoms don't share electrons easily, the cohesive forces between mercury atoms are weak, causing it to melt at a very low temperature of −38.83 

∘

 C.

Quick Facts About the Other Options:

(B) Zinc: A solid, bluish-white transition metal commonly used to galvanize iron and steel to prevent rusting.

(C) Arsenic: A solid metalloid (having properties of both metals and non-metals) famously known for its toxicity.

(D) Tungsten: A solid metal with the highest melting point of all pure metals (3422 

∘

 C), traditionally used in incandescent light bulb filaments.

Note: Bromine is another element that is liquid at room temperature, but it is a non-metal. If a question asks for a liquid non-metal, Bromine is your answer; for a liquid metal, it's always Mercury.