The degree of unsaturation of lipids can be measured as

The correct answer is Iodine number.

Step-by-Step Explanation:

1. Understanding Unsaturation in Lipids:
Unsaturation in lipids (fats and oils) refers to the presence of double bonds between carbon atoms (C=C) in their fatty acid chains. Saturated lipids contain only single bonds, whereas unsaturated lipids contain one or more double bonds. The degree of unsaturation is a measure of how many double bonds are present in the lipid structure.

2. How Iodine Number Measures Unsaturation:
Halogens like iodine can easily add across the carbon-carbon double bonds of unsaturated fatty acids in a chemical reaction known as halogenation. The reaction can be generalized as:
-CH=CH- + I₂ → -CHI-CHI-
Since each double bond reacts with one molecule of iodine, the amount of iodine consumed by a fat or oil is directly proportional to the number of double bonds (degree of unsaturation) present in the lipid. The Iodine number (or iodine value) is defined as the number of grams of iodine absorbed by 100 grams of the fat or oil.

3. Why Other Options are Incorrect:
- Saponification number: This measures the average molecular weight (or chain length) of all the fatty acids present in the fat. It is defined as the number of milligrams of potassium hydroxide (KOH) required to saponify 1 gram of fat.
- Reichert Meissel number: This measures the amount of steam-volatile, water-soluble fatty acids (primarily butyric and caproic acids) present in a fat.
- Polenske number: This measures the amount of steam-volatile, water-insoluble fatty acids (such as caprylyl, capryl, and lauryl acids) present in a fat.