Final answer:
The particle in Earth's atmosphere with the fastest speed is nitrogen molecules (N₂).
Step-by-step explanation:
The particle in Earth's atmosphere that you would expect to have the fastest speed is A. N₂, which represents nitrogen molecules.
Based on the information provided, cosmic rays are particles found in interstellar space that travel at high speeds. However, the question specifically refers to particles in Earth's atmosphere, so we must consider the particles in that context. Nitrogen molecules (N₂) have a lower molecular weight compared to the other options (O₂, CO₂, Ar), which means they have a higher root mean square (rms) speed at a given temperature. Therefore, nitrogen molecules are expected to have the fastest speed in Earth's atmosphere.