Which of the following statements regarding electric insulators is correct?

• A. Electric insulators can conduct electricity as efficiently as metals.
• B. Electric insulators become charged only when connected to a battery.
• C. Electric insulators have a large number of free electrons that move easily.
• D. Electric insulators do not allow free movement of electric charges within them.

Answer: (D)

Verified step by step guidance

1. Understand the definition of an electric insulator: materials that do not allow free movement of electric charges within them.
2. Recall that metals are good conductors because they have many free electrons that can move easily, unlike insulators.
3. Recognize that insulators do not conduct electricity efficiently, so the statement 'Electric insulators can conduct electricity as efficiently as metals' is incorrect.
4. Note that insulators can become charged by friction or other means, not only when connected to a battery, so the statement about charging only when connected to a battery is incorrect.
5. Conclude that the correct statement is: 'Electric insulators do not allow free movement of electric charges within them,' which explains their inability to conduct electricity.