Which of the following types of polymers is not based on the classification by the source?

Elastomers

The correct answer is option 3. Elastomers.

The type of polymer "Elastomers" is not based on the classification by the source.

Here is a breakdown:

1. Natural: Natural polymers are derived from natural sources such as plants, animals, or microorganisms. Examples include proteins, cellulose, and natural rubber.

2. Semi-synthetic: Semi-synthetic polymers are derived from natural polymers but undergo chemical modifications to enhance their properties or create new materials. Examples include cellulose derivatives like cellulose acetate and cellulose nitrate.

3. Elastomers: Elastomers are a class of polymers characterized by their elastic properties, meaning they can stretch and return to their original shape when released. Elastomers can be either natural or synthetic and are not classified based on their source but rather their mechanical properties.

4. Synthetic: Synthetic polymers are entirely man-made and are produced through chemical reactions from raw materials derived from petroleum, natural gas, or other sources. Examples include polyethylene, polypropylene, and polystyrene.

So, the classification "Elastomers" does not fall under the source-based classification but rather categorizes polymers based on their mechanical properties.