When a fish expands its air bladder, the density of the fish
decreases. increases. remains the same. none of the above

Every substance, body or material has mass and volume, however the mass of different substances occupy different volumes.

This is where density    appears as a  physical characteristic property of matter that establishes a relationship between the mass   of a body or substance and the volume   it occupies.

So, according to this equation, the density is inversely ptoportional to the volume:

If the volume increases, the density decreases.

This is what a fish does to have buoyancy, since the density of a body is related to its buoyancy:

A body will float on another fluid if its density is lower.