Which phase of matter has a fixed volume, but variable shape?

solid

liquid

gas

plasma

Answer: liquid

Explanation: a liquid has a volume that does not change (unless you add more or less liquid to it, obviously) but its shape can change depending on the container you use.

Think, if you put a liter of water into a bucket, it will still be a liter of water if you pour it into a water bottle. The only thing different is that the shape of the liquid changes.