Final answer:
Carbon dioxide (C) would diffuse the slowest among the given options. Carbon dioxide has a higher molar mass compared to radon, sulfur hexafluoride, and hydrogen, so it will diffuse the slowest among the given options.
Step-by-step explanation:
The gas that would diffuse the slowest is carbon dioxide (C).
The rate of diffusion of a gas depends on its molar mass.
The heavier the gas, the slower it will diffuse.
Carbon dioxide has a higher molar mass compared to radon, sulfur hexafluoride, and hydrogen, so it will diffuse the slowest among the given options.