ANS:D - is incompressible & frictionless.

When referring to a fluid being "incompressible and frictionless," we are describing ideal fluid behavior under certain conditions. Let's break down these terms:

1. Incompressible: An incompressible fluid is one in which the density remains constant regardless of changes in pressure. In other words, when a force is applied to an incompressible fluid, it does not change volume or density. This assumption simplifies fluid mechanics calculations and is often applicable to liquids like water under certain conditions.
2. Frictionless: A frictionless fluid, also known as an inviscid fluid, is a fluid that has no viscosity. Viscosity is the property of a fluid that causes internal resistance to flow. In a frictionless fluid, there is no resistance to flow, and the fluid particles can move without any energy loss due to friction.

When we say "is incompressible and frictionless," we are typically referring to ideal fluid behavior, which serves as a simplifying assumption in many fluid dynamics problems. It allows engineers and scientists to analyze fluid flow patterns and pressure distributions in a straightforward manner, although it may not fully represent real-world fluid behavior.