Which outer layer is indicated by 'N'?

The correct outer layer indicated by 'N' is nylon. Nylon is a synthetic polymer that is widely used in electrical insulation due to its excellent dielectric properties and resistance to abrasion. It is often chosen for wire insulation and other electrical applications because of its ability to withstand heat and chemicals, while providing a protective barrier against moisture and environmental factors. The versatility and strength of nylon make it an ideal choice for various electrical components, enhancing both safety and durability in power production systems.

While other materials listed may also be used in insulation and protective applications, nylon is specifically recognized for its unique properties that cater to the demands of electrical wiring and equipment.