The correct answer is A. glass.
Porcelain is a type of ceramic made from kaolin, silica, and feldspar. It is a hard, brittle material that is often used for insulators because it is resistant to electricity and heat. Glass is also a hard, brittle material that is resistant to electricity and heat. However, it is less expensive than porcelain and easier to work with. This makes it the preferred material for most transmission line insulators.
Iron is a metal that is not resistant to electricity. This means that it would not be a good material for transmission line insulators, as it would allow electricity to flow through it.
P.V.C. is a type of plastic that is not resistant to electricity. This means that it would not be a good material for transmission line insulators, as it would allow electricity to flow through it.
In conclusion, the correct answer is A. glass.