Final answer:
Temperature is an extraneous variable that can greatly affect the rate of diffusion, as it influences the kinetic energy of the molecules, whereas surface area and concentration gradient are primary factors in the process of diffusion.
Step-by-step explanation:
Among the options provided, surface area, concentration gradient, and the mass of the molecules diffusing are factors directly related to the rate of diffusion. These factors, however, are not considered to be extraneous variables in the context of diffusion, as they are integral to its process. Extraneous variables are those that are not intentionally studied. In the provided context, temperature, pressure, and solution density are examples of extraneous variables that could affect the rate of diffusion, with temperature being one of the most significant.
For example, an increase in temperature generally causes molecules to move more rapidly this is because they have more kinetic energy, thus they diffuse faster. It's important to distinguish these key variables that can potentially influence the experimental outcome but are not the primary variables under examination.
When experimenting or observing diffusion, controlling these extraneous factors is essential to ensuring accurate and relevant results, separating them from the key variables such as the concentration gradient or molecular size.