On which type of matter, the effect of pressure is the highest?

The effect of pressure is typically highest on gases, compared to liquids or solids, due to their low density and high compressibility. When pressure is applied to a gas, its particles are forced closer together, reducing the volume of the gas.

So, The effect of pressure on solids is very negligible as they have particles closely packed together. Effect of pressure is significant only on liquids and gases and highest effect is seen in gases as they have large intermolecular spaces.

Hence option C is correct.