The material used for making the heating element of electrical appliances is
A) aluminium
B) copper
C) steel
D) nichrome

Hint: When the current is passed on the electrical conducting wire, the heat is generated on the wire. The heating element converts the electrical energy into the heat energy by the process of the Joule heating. To resist the generation of the wire the heating element is used. The direction of the current is independent for this process.

Complete step by step solution:
This heating element may be wire or ribbon, straight or coiled. Generally, they are used in common heating devices like bread toasters, hair dryers, furnaces for the stoves etc. The most common material used for the heating element is Nichrome. Nichrome is the alloy of the Nickel and chromium; in this alloy the nickel is $80\% $ and the chromium is $20\% $. So, this alloy is called nichrome $80/20$. It is an ideal material, because it has a high resistance to the heat and forms the layer of chromium oxide, when the nichrome is heated for the first time. After the formation of the layer of the chromium oxide, the material beneath the layer of chromium oxides will not oxidize, so it prevents the wire from heating. If the wire is more heated, it leads to the breaking of the wire or the burning out of the wire.

Hence, the option (D) is the correct answer.

Note: The nichrome is used to resist the heat, and the insulating material is used to cover the nichrome wire, it is secondary insulation to resist the heat formation of the wire by the external heat. And also, to resist the breaking of the wire due to heat.