Verified answer

water , H2O and carbon dioxide,CO2 combine to form sugar. Sugar is a compound because it can be broken down into simpler substances.

Sugar has a formula C6H12O6 showing that it is composed of 3 elements

An iron nail rusting in air seems to be decaying, but is actually combining with the oxygen in the air to form iron oxide or rust.

Conservation of mass shows that you have the same kind and amount of elements on both sides of the arrow. 4Fe+3O2---> 2Fe2O3

You know it's not an element because when you heat it, you get carbon and water.

How do you know it's a single compound? Good question. Unchanged if you try separation techniques like crystallising or chromatography. All the crystals look the same and give same xrd diffraction pattern. And same infrared spectrum. But I think that's beyond the level you re being asked

The rusting nail appears to violate conservation of mass, because the rust weighs more than the nail. I think you are being asked where the extra mass is coming from. But not very easy to set up an expt to prove it.

A compound is a molecule containing more than one type of atom. Carbon, hydrogen, and oxygen makes more than one.

because , wow you are that stupid