Explanation:

Here’s the thing—let’s go through each choice, straight and to the point:

- Option 1: Hydrometer

Measures the density (or specific gravity) of liquids. Think chemistry labs, not music or vibrations.

- Option 2: Sonometer

This is the one. Sonometers are made to study how strings vibrate, their tension, length, frequency—all that good physics stuff.

- Option 3: Sphygmomanometer

That’s a fancy word for the device doctors use to check your blood pressure. Absolutely nothing to do with vibrating strings.

- Option 4: Electrometer

Measures electric charge or potential difference. Not used for anything related to musical strings.

So you picked Option 2—Sonometer.