Which of the following instrument is used to measure gauge pressure ?

Options

A. Thermometer
B. Barometer
C. Manometer
D. Hydrometer

Explanation

To determine which instrument is used to measure gauge pressure, let's analyze the options provided:

1. **Thermometer**: This instrument is specifically designed to measure temperature. It does not measure pressure.

2. **Barometer**: A barometer is used to measure atmospheric pressure. It measures the pressure exerted by the atmosphere and is not used for measuring gauge pressure.

3. **Manometer**: A manometer is an instrument specifically designed to measure the pressure of a fluid relative to atmospheric pressure, which is known as gauge pressure. It can be used to measure the pressure of gases or liquids in various applications.

4. **Hydrometer**: A hydrometer is used to measure the relative density (or specific gravity) of liquids. It does not measure pressure.

Based on the analysis of the options, the correct answer is:

**Manometer** - it is the instrument used to measure gauge pressure.

### Step-by-Step Solution:
1. **Identify the Instruments**: List the instruments provided in the question: Thermometer, Barometer, Manometer, Hydrometer.

2. **Analyze Each Instrument**:
- **Thermometer**: Measures temperature.
- **Barometer**: Measures atmospheric pressure.
- **Manometer**: Measures gauge pressure.
- **Hydrometer**: Measures relative density of liquids.

3. **Determine the Purpose of Each Instrument**: Understand that gauge pressure is the pressure relative to atmospheric pressure, which is specifically measured by a manometer.

4. **Select the Correct Instrument**: Based on the analysis, the manometer is identified as the instrument that measures gauge pressure.

5. **Conclude**: The answer to the question is the manometer.