Which method is appropriate for separating a mixture of iron and aluminum

Using a magnet? It would collect iron, but not aluminum.

You would use a magnet, this is because the magnet would pick up the iron and not the aluminum.
Magnets attract various metals and alloys, including iron, but not including aluminum, so if you put the magnet above a mixture of iron and aluminum, it will pick up the iron filings and stick to them, but it won't do that to the aluminum, so when you take the magnet away it will drag the iron but not the aluminum.

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2