Which of the following materials is an electric insulator?
A) Aluminium
B) Gold
C) Rubber
D) all of these

Hint: An electric insulator is a material which exhibits high resistivity to the flow of current through it. They have strongly bound electrons in their atoms, which do not tend to move freely to facilitate the flow of electricity. The most common examples of insulators are non-metals.

Complete answer:
Electric insulators are materials with strongly bound electrons in their atoms. Due to this, charges present in the atoms of these materials are not free to move randomly and thus, they do not tend to conduct electricity easily. Clearly, insulators are said to have high resistivity to the flow of electric current. The most common examples of insulators are non-metals. Some other good insulators are paper, Teflon and glass.
Non-metals are the materials which have entirely different properties than those of metals. For example, most of the non-metals have low melting-point, boiling point, density and poor thermal conductivity. As already mentioned, they also have poor electrical conductivity. Some of the examples of non-metals are oxygen(gas), bromine(liquid), carbon(solid) and germanium(metalloid).
Out of the given options, we know that aluminium and gold are metals whereas rubber is a non-metal. Therefore, we can conclude that rubber is the electric insulator among the given options.

Hence, the correct answer is option C.

Note:
We can also deduce the answer from real-life situations like the coating of electric wiring, we use at our homes. Clearly, these electric wires are coated using rubber-like polymers to ensure proper insulation properties. Talking about insulators, it is to be noted that under suitable conditions such as high temperature or in the presence of high voltage, insulators may suffer dielectric breakdown and become conductors.