At STP (Standard Temperature and Pressure), the gas that diffuses most rapidly is hydrogen gas (H₂).

Here's why:

* Graham's Law of Diffusion: This law states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

* Molar Mass: Hydrogen gas has the lowest molar mass among all gases (2 g/mol).

* Lower Molar Mass = Faster Diffusion: A lower molar mass means the gas molecules are lighter and move faster, leading to a faster rate of diffusion.

Therefore, due to its low molar mass, hydrogen gas diffuses the fastest at STP.