Gas is the state of matter with a fixed volume that can be compressed slightly. Let's break down this concept and understand why the correct answer is option 'C'.

Gases are one of the three main states of matter, along with solids and liquids. Unlike solids and liquids, gases do not have a fixed shape or volume. They occupy the entire space available to them, filling up any container they are placed in. However, they do have a property called compressibility, which allows them to be compressed or squeezed into a smaller volume.

Here's why gases have a fixed volume that can be compressed slightly:

1. Definition of a gas:
- A gas is a state of matter that consists of particles (atoms or molecules) that are in constant motion and have a lot of space between them.
- These particles move rapidly and randomly, colliding with each other and the walls of their container.

2. Volume of a gas:
- Gases do not have a fixed volume because they can expand or contract to fill the available space.
- The volume of a gas is determined by the size of its container.

3. Compressibility of gases:
- Gases can be compressed because of the large amount of empty space between particles.
- When a gas is compressed, its particles are forced closer together, reducing the volume it occupies.
- This compression is possible because the particles in a gas have enough kinetic energy to allow them to move closer together.

4. Limitations on compression:
- While gases can be compressed, there are limitations to how much they can be compressed.
- At very high pressures, the particles in a gas are forced extremely close together, and intermolecular forces start to play a significant role.
- These intermolecular forces resist further compression and eventually prevent the gas from being compressed any further.

In conclusion, the state of matter with a fixed volume that can be compressed slightly is a gas. Gases have a variable volume, as they can expand or contract to fill any container. However, they can be compressed due to the large amount of empty space between particles. This property of gases makes them different from solids and liquids, which have fixed volumes and limited compressibility.