ANS:D - none of these.

In fluid dynamics, the term "steady flow" refers to a flow regime where the fluid properties, such as velocity, pressure, density, and temperature, do not change with respect to time at any given point in the flow field. This means that the flow variables remain constant over time at a fixed location within the flow. Here's a more detailed explanation of why each of the variables—velocity, pressure, and density—remains constant in steady flow:

1. Velocity: In steady flow, the velocity of the fluid particles at any point in the flow field remains constant over time. This implies that the fluid particles move with a consistent speed and direction at any given location within the flow. There is no acceleration or deceleration of the fluid particles with time.
2. Pressure: Similarly, the pressure at any point in the flow field remains constant with respect to time in steady flow. Pressure is a measure of the force exerted by the fluid on the walls of its container or on any object immersed in the fluid. In steady flow, the distribution of pressure throughout the flow field remains unchanged over time.
3. Density: Density is a measure of how much mass is contained in a unit volume of a substance. In steady flow, the density of the fluid remains constant at any given point in the flow field over time. There is no change in the concentration of fluid particles within the flow, so the density remains uniform.

In summary, steady flow is characterized by the absence of changes in velocity, pressure, and density with respect to time at any fixed point in the flow field. This steady-state condition simplifies the analysis of fluid flow problems and allows for the application of conservation laws, such as the conservation of mass, momentum, and energy, to describe and predict fluid behavior accurately.