The most commonly used material for insulators in transmission lines:
Correct answer: 3. Porcelain
Explanation: Porcelain is the most commonly used material for insulators in transmission lines.
Glass insulators were once the most popular type of insulators. However, they fail in high temperatures (Since the coefficient of thermal expansion of glass is low). Due to this reason the electrical power systems usually employ porcelain insulators.
With the advancement in chemical engineering, the Composite polymer insulators are also finding their applications, however, they have high leakage current and their life span is also comparatively short due to which the porcelain insulators are the ultimate chance for the majority of high voltage power transmission and distribution systems.