Which one of the following is the hardest metal?

Answer: Option D

Tungsten is the hardest metal among the given options.

It has the highest melting point (about 3422°C) of all pure metals.

Its hardness and ability to withstand high temperatures make it widely used in the production of electrical filaments, cutting tools, and rocket engine nozzles.

Gold is a soft metal, Iron is relatively strong but not the hardest, and Platinum is dense and corrosion-resistant but not as hard as Tungsten.

Therefore, the hardest metal is Tungsten.