Which of the following metals exists as liquid at room temperature? 

Mercury 

The correct answer is option 4. Mercury.

Among the options provided, mercury is the metal that exists as a liquid at room temperature. Mercury (Hg) is a unique metal that remains in a liquid state at standard room temperature and atmospheric pressure. It is the only metal that exists in a liquid form under these conditions. Mercury has a low melting point of -38.83°C (-37.89°F) and a boiling point of 356.7°C (674.1°F), allowing it to remain liquid over a wide range of temperatures commonly encountered on Earth's surface. So, the correct answer is Mercury.