Flow classification by Mach number: if the Mach number M for a fluid flow is less than 1, the flow is termed as:

Introduction / Context:
Compressible-flow regimes are categorized by the Mach number M = V / a, where V is flow speed and a is the local speed of sound. Correct regime identification guides which physical effects and equations are important (e.g., shock waves, compressibility corrections).

Given Data / Assumptions:

• Single stream of a gas in a duct or external flow.
• Local Mach number M < 1.

Concept / Approach:
Standard ranges: subsonic (M < 1), sonic (M = 1), supersonic (M > 1 up to ~5), hypersonic (M ≳ 5), transonic (M near 1, typically 0.8–1.2). For subsonic flows, traditional incompressible methods can be adequate when M ≲ 0.3–0.5, with corrections as M increases toward unity.

Step-by-Step Solution:
Compare given M with regime thresholds.Since M < 1, the correct term is “subsonic.”

Verification / Alternative check:
Textbook charts of compressible flow consistently adopt these definitions and boundaries.

Why Other Options Are Wrong:
Sonic: applies only at M = 1.Supersonic / hypersonic: require M > 1, and much greater for hypersonic.Transonic: refers to around-unit Mach with mixed sub/supersonic regions.

Common Pitfalls:

• Equating “incompressible” with all M < 1; in practice, incompressible assumptions are usually valid near M ≲ 0.3.
• Overlooking temperature variation effects on the local speed of sound.

Final Answer:
Subsonic