The image represents a human cell in an isotonic solution some common cellular molecules that can freely move across the plasma membrane are shown.

In an isotonic solution, the concentration of solutes outside the cell is equal to the concentration of solutes inside the cell. This means that there is no net movement of water into or out of the cell.

Considering the options provided:

A. Some CO2 will leave the cell - CO2 generally diffuses out of the cell due to a higher concentration inside (as a product of cellular respiration), so this would occur.

B. Some O2 will enter the cell - Oxygen diffuses into the cell, but only if the concentration is higher outside the cell. In an isotonic environment, it depends on O2 availability outside.

C. Some H2O will enter the cell - In an isotonic solution, there is no net movement of water into the cell.

D. Some H2O will leave the cell - Again, in an isotonic solution, there is no net movement of water out of the cell.

Given the above reasoning, the best answer in terms of what will definitely happen if no energy is used would be:

**A. Some CO2 will leave the cell.**

This is because CO2 can diffuse out of the cell without the need for energy, assuming a concentration gradient exists.