Purity of milk is tested by :

Correct option is B

The purity of milk is tested using a lactometer, an instrument used to measure the density of milk to detect its purity. Milk’s density changes if it's diluted or adulterated. A lactometer floats in the milk, and based on its buoyancy, it can determine the density, indicating how pure the milk is.

Important Key Points:

1. Lactometers are calibrated to show the density of pure milk.
2. A high reading indicates denser, possibly adulterated milk.
3. Water adulteration lowers the milk's density, which the lactometer detects.
4. Hydrometers are similar but used for measuring liquid densities in other contexts.
5. Milk adulteration is a major concern for public health, and lactometers help safeguard quality.
6. Lactometers are easy to use and cost-effective for quality assurance.

Knowledge Booster:

• Hydrometer: Measures the relative density of liquids.
  
• Thermometer: Measures temperature, not the density of liquids like milk.