Thermosetting Plastic
Thermosetting plastics, also known as thermosets, are a type of polymer that undergoes a chemical reaction during the curing process and becomes permanently hardened. Once they are formed, they cannot be melted or reshaped like thermoplastics. This characteristic makes them highly durable and resistant to heat and chemicals.

Examples of Thermosetting Plastics
There are several examples of thermosetting plastics, including polythene, dacron, and nylon.

1) Polythene: Polythene is not a thermosetting plastic. It is a type of thermoplastic that can be melted and reshaped multiple times without losing its properties. Thermoplastic polymers have weaker intermolecular forces, allowing them to be heated, molded, and cooled repeatedly.

2) Dacron: Dacron, also known as polyethylene terephthalate (PET), is a thermoplastic polymer commonly used in the production of clothing fibers, plastic bottles, and other products. It can be melted and reshaped, so it is not a thermosetting plastic.

3) Nylon: Nylon is another example of a thermoplastic polymer. It is widely used in the production of synthetic fibers, fabrics, and various types of plastics. Like polythene and dacron, nylon can be melted and reshaped, making it a thermoplastic rather than a thermosetting plastic.

4) All of these: None of the options provided in the question (polythene, dacron, and nylon) are examples of thermosetting plastics. Therefore, the correct answer is none of the above.

Conclusion
Thermosetting plastics are a distinct category of polymers that undergo chemical reactions during the curing process, resulting in hardened and irreversible structures. Examples of thermosetting plastics include materials like epoxy, phenolic resins, and melamine. However, in the options provided, none of the examples (polythene, dacron, and nylon) qualify as thermosetting plastics.