The correct statement is:
D. O2 is a molecule while CO2 is a compound.
Here's a brief explanation:
- O2 is a molecule composed of two oxygen atoms, but it is not a compound since it consists of only one element (oxygen).
- CO2 is a compound because it consists of two different elements (carbon and oxygen) combined in a fixed ratio.
Therefore, option D accurately describes the classification of these two substances.