The correct statement about plasma membranes is B: both plant cells and animal cells have flexible plasma membranes.
To clarify the other options: A. Plant cells have cell walls in addition to plasma membranes, not instead of them. C. Plant cells do not have "Richard" in their plasma membranes; this statement seems to be unclear or incorrect. D. Animal cells have plasma membranes, not "cell membranes" as a distinct different structure.
So, option B is the most accurate statement.