The correct statement is: Both plant cells and animal cells have flexible plasma membranes.
To clarify the other options:
- Plant cells do have a cell wall, but they also have a plasma membrane. The cell wall provides rigidity, while the plasma membrane is flexible.
- The terms "cell membrane" and "plasma membrane" are often used interchangeably for animal cells; hence, it's misleading to say animal cells have cell membranes instead of plasma membranes.
- Plant cells do not have rigid plasma membranes; rather, their plasma membranes are flexible, but the cell wall provides rigidity.
Therefore, the first statement is the most accurate.