ANS:D - less than 0.2

The Mach number (Ma) is a dimensionless quantity representing the ratio of the speed of an object (or fluid) to the speed of sound in the medium through which the object is moving. It is defined as: Ma=v/c​ where:

• v is the velocity of the object (or fluid), and
• c is the speed of sound in the medium.

The Mach number is used to determine the compressibility effects of a fluid flow. In the case of gases, if the Mach number is significantly less than 1, it indicates that the flow is incompressible and the compressibility effects can be neglected. Therefore, if we want gases to be considered incompressible, the Mach number would typically be much less than 1. Among the given options, "less than 0.2" would be the closest, although the exact threshold may vary depending on the specific application and definition of "incompressible."