The correct answer is A. incompressible.
An ideal fluid is a fluid that is inviscid and incompressible. This means that it has no viscosity, or internal friction, and it cannot be compressed. Ideal fluids are often used in theoretical models, as they are much simpler to analyze than real fluids.
A viscous fluid is a fluid that has viscosity, or internal friction. This means that there is a resistance to the flow of the fluid, due to the friction between the molecules of the fluid. Viscous fluids are more difficult to analyze than ideal fluids, but they are more realistic.
A compressible fluid is a fluid that can be compressed. This means that the volume of the fluid can be reduced by applying pressure. Compressible fluids are more difficult to analyze than incompressible fluids, but they are more realistic.
In conclusion, an ideal fluid is a fluid that is inviscid and incompressible. This means that it has no viscosity, or internal friction, and it cannot be compressed. Ideal fluids are often used in theoretical models, as they are much simpler to analyze than real fluids.