Step 1: Understand the definition of an insulator.
An electrical insulator is a material that does not allow electric current to flow through it easily. Insulators have very high electrical resistance. Step 2: Analyze the electrical conductivity of the given materials.
Copper: Copper is a metal and a very good conductor of electricity due to the presence of free electrons.
Silver: Silver is also a metal and an excellent conductor of electricity, even better than copper in terms of conductivity.
Aluminium: Aluminium is another metal and a good conductor of electricity, commonly used in power transmission lines.
Rubber: Rubber is a polymer and a poor conductor of electricity. It has very few free electrons, making it an excellent electrical insulator.
Step 3: Identify the insulator among the given options.
Based on their electrical conductivity properties, rubber is an insulator, while copper, silver, and aluminium are conductors.