Final answer:
Air cannot be compressed, while water and brake fluid can be compressed to some extent under high pressure.
Step-by-step explanation:
Out of the given options, air is the only one that cannot be compressed.
Air is a mixture of gases and its compressibility is determined by the properties of those gases. The particles that make up air have a certain volume and can be compressed under high pressure. On the other hand, both water and brake fluid are fluids and can be compressed to some extent under high pressure.
Therefore, the correct answer is option A: Air cannot be compressed.
Learn more about Compressibility of Air, Water, and Brake Fluid