The correct answer is option-2, rust is a compound.

To understand this, we need to know what rust is. Rust is a reddish-brown substance that forms on the surface of iron or steel when it is exposed to oxygen and moisture in the air.

Option-1 suggests that rust is an element, but rust is actually a compound made up of multiple elements. It is primarily composed of iron oxide, which is a compound formed by the reaction of iron, oxygen, and water.

Option-3 suggests that rust is a mixture of iron and dust. While dust particles can accumulate on the surface of rust, they are not an essential component of rust itself.

Option-4 is the correct option. Rust is formed by a chemical reaction between iron, oxygen, and water. This reaction is called oxidation. When iron is exposed to air and moisture, it reacts with oxygen to form iron oxide, which is the compound we know as rust.

In conclusion, rust is a compound formed by the reaction of iron, oxygen, and water.