Which of the following materials is an electrical insulator, that is, a substance that does not allow electric current to pass through it easily?

Introduction / Context:
Electrical conductors and insulators play a central role in circuits, wiring, and safety. Conductors allow electric current to flow easily, while insulators resist current flow and are used to protect people and equipment. This question asks which of the listed substances is an insulator, meaning it is a poor conductor of electricity under normal conditions.

Given Data / Assumptions:

• We consider electrical behaviour at room temperature and normal conditions.
• Metals are generally good conductors of electricity.
• Non metallic materials like dry wood are usually insulators.
• The options include several metals and one non metal material.

Concept / Approach:
In metals such as copper, aluminium, mercury and iron, there are free electrons that can move easily through the lattice, making these substances good conductors of electricity. In contrast, in materials like dry wood, electrons are not free to move, so they do not conduct electric current well. Such materials are used as insulators in electrical equipment. Therefore, identifying wood as the insulator among mostly metallic options demonstrates understanding of conductor versus insulator behaviour.

Step-by-Step Solution:

Verification / Alternative check:
Everyday examples support this conclusion. Electrical wires are usually made of copper or aluminium because they conduct electricity very well, but they are covered with insulating materials such as plastic or rubber. Wooden poles have historically been used to support power lines because wood does not conduct electricity effectively when dry. Mercury is used in some switches because it conducts, not because it insulates. These practical uses confirm which materials behave as conductors or insulators.

Why Other Options Are Wrong:

• Copper: A highly efficient electrical conductor widely used in wiring and electrical components.
• Mercury: A metal that is liquid at room temperature but still conducts electricity well.
• Aluminium: A light metal used in power lines due to its good conductivity.
• Iron: A metal used in electrical machines and structures and is also a conductor, although not as good as copper or aluminium.

Common Pitfalls:
Students may occasionally be misled by the physical state of mercury and think that because it is a liquid, it behaves differently from other metals. However, it remains a conductor. Another mistake is to think that slightly damp wood can conduct electricity and then generalise that wood is always a conductor. In standard theory, dry wood is treated as an insulator, which is the context of the question.

Final Answer:
The material that acts as an electrical insulator is Wood.