**Insulator:** An insulator is a material that does not allow the flow of electric current easily. It has high electrical resistance and low conductivity.

**Options:**

a) Graphite: Graphite is not an insulator. It is a good conductor of electricity due to the presence of free electrons in its structure.

b) Air: Air is an insulator. It does not conduct electricity well because its atoms and molecules do not have free electrons to carry electric charge.

c) Aluminium rod: Aluminium is a good conductor of electricity, so it is not an insulator. It has a high number of free electrons, which allows the flow of electric current.

d) Copper rod: Copper is also a good conductor of electricity, so it is not an insulator. It has a high number of free electrons, which enables the flow of electric current.

**Explanation:**

Air is an excellent example of an insulator. Here's why:

1. **Lack of Free Electrons:** Air is made up of molecules, which consist of tightly bound atoms. These atoms do not have free electrons that can move around and carry electric charge. Therefore, air does not conduct electricity well.

2. **High Electrical Resistance:** Electrical resistance is a measure of how difficult it is for electric current to flow through a material. Insulators have high electrical resistance. Air has a high resistance due to the absence of free electrons that can easily carry electric charge.

3. **Breakdown Voltage:** Another characteristic of insulators is their ability to withstand high voltages without breaking down. Air has a high breakdown voltage, which means it can resist the flow of electric current even under high voltage conditions. This property makes it suitable for use as an insulator in various applications, such as insulating wires and cables.

Overall, air is an insulator because it lacks free electrons, has high electrical resistance, and can withstand high voltages without breaking down.

Air is a poor conductor or insulator because it does not allow the heat and electricity to pass through them.