Bulk Modulus describes how resistant a substance is to compression. It is defined as the change in the relative volume of an object when stress is uniformly applied to the surface of the object. It is the ratio of volumetric stress to volumetric strain and is denoted by B.

So, bulk modulus is related to matter(solid, liquid or gas), it is related to any volume. The question is how compressible the matter is. 

solids have least compressibility < liquids are more compressible than solid < gases show maximum compressibility.

Thus solids are having the greatest Bulk Modulus.