Which of the following statements is correct about diffusion? a. It is very rapid over long distances. b. It requires an expenditure of energy by the cell. c. It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration. d. It is an active process in which molecules move from a region of lower concentration to one of higher concentration. e. It requires integral proteins in the cell membrane.

To determine which statement about diffusion is correct, let's analyze each option: a. **It is very rapid over long distances.** - This statement is incorrect. Diffusion is generally a slow process, especially over long distances. The rate of diffusion decreases as the distance increases. b. **It requires an expenditure of energy by the cell.** - This statement is incorrect. Diffusion is a passive process and does not require energy expenditure by the cell. c. **It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration.** - This statement is correct. Diffusion is indeed a passive process where molecules move down their concentration gradient, from areas of higher concentration to areas of lower concentration. d. **It is an active process in which molecules move from a region of lower concentration to one of higher concentration.** - This statement is incorrect. This describes active transport, not diffusion. Diffusion does not require energy and moves from high to low concentration. e. **It requires integral proteins in the cell membrane.** - This statement is incorrect for simple diffusion. While facilitated diffusion does require integral proteins, simple diffusion does not. Based on this analysis, the correct statement about diffusion is: **c. It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration.**