Explanation:

- Option 1: A mixture of aluminium and copper

- Aluminium and copper are excellent conductors of electricity, but they are typically not used to make coils for water heaters. They don't have the necessary resistance for efficient heating.

- Option 2: A mixture of iron and steel

- Iron and steel can conduct electricity. However, they are prone to rust and may not provide the required resistance and durability for heating applications.

- Option 3: Nichrome (Correct Answer)

- Nichrome is an alloy commonly used in heating elements. It consists mainly of nickel and chromium. Its high resistance and ability to withstand high temperatures make it ideal for this purpose.

- Option 4: Copper

- While copper is an excellent electrical conductor, it is rarely used in heating elements because of its low resistivity, leading to insufficient heating.