**Core Concept**
A Venturi meter, also known as a Venturi tube, is a device used to measure the flow rate of fluids, particularly in gas-liquid systems. It works on the principle of Bernoulli's equation, which states that the pressure of a fluid decreases as its velocity increases.

**Why the Correct Answer is Right**
The Venturi meter is specifically designed to create a region of low pressure by constricting the flow area, resulting in an increase in fluid velocity. This increase in velocity is directly proportional to the decrease in pressure, allowing for the measurement of the flow rate. The device is commonly used in medical settings to measure the flow rate of gases, such as oxygen or anesthetic gases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a manometer is a device used to measure pressure, not flow rate.
**Option B:** This option is incorrect because a flowmeter is a more general term that encompasses various devices used to measure flow, but it does not specifically refer to a Venturi meter.
**Option C:** This option is incorrect because a spirometer is a device used to measure lung volumes, not flow rates.

**Clinical Pearl / High-Yield Fact**
A Venturi meter is a high-precision device that is often used in medical settings to measure the flow rate of gases, such as oxygen or anesthetic gases, which is essential for maintaining patient safety and optimizing treatment outcomes.

**Correct Answer:** D.