The best description of a physical state of matter that has a fixed volume but takes the shape of its container is Liquid.

Here's why:

* Solids have a fixed volume and a fixed shape.

* Gases take the shape and volume of their container.

* Semi-solids (or viscoelastic materials) exhibit properties of both solids and liquids. They can deform under stress but can also maintain their shape.

Liquids are the only state of matter that fits the given description: they maintain a constant volume but readily adapt to the shape of their container.