Answer:
I believe A is the correct answer.
Explanation:
I note that another response includes C. as another correct answer. It is possible, but it depends on what is meant by "fixed shape." I interpret "fixed shape" as the collection of the gases forming a fixed shape on their own. Gases fill the container they are in (be it a flask, room, or a planet, so they have a fixed shape by that definition. But a primary definition of gases is that that take on the shape of their container, not a shape of their own.
A. is correct because perfect gases have no interactions (AKA the Ideal Gas Laws). They may have interactions, and often do, but they are weak. That's why the ideal gas laws are a good first approximation to how gases change volume based on temperature and pressure.
