Corundum is an ore of:

aluminium

The correct answer is option 4. aluminium.

Corundum is a mineral with the chemical formula Al₂O₃, meaning it consists of aluminum (Al) and oxygen (O) atoms. It is a crystalline form of aluminum oxide. The characteristic that makes corundum valuable in various applications is its hardness, making it one of the hardest minerals.

Now, let's look at the options:

1. Copper:  Corundum is not an ore of copper. Copper ores typically include minerals like chalcopyrite or bornite.

2. Silver:  Corundum is not an ore of silver. Silver ores usually contain minerals like argentite or galena.

3. Iron:  Corundum is not an ore of iron. Iron ores commonly include hematite or magnetite.

4. Aluminium:  This is the correct answer. Corundum is indeed an ore of aluminum. When people mine corundum, they are essentially extracting aluminum oxide, which can be further processed to obtain aluminum.

So, the answer is 4. aluminium.