Why is it possible to use physical properties to separate a mixture

It is possible to use physical properties to separate a mixture- at least of solids. For example, in a mixture of sand and magnetite, the magnetite can be separated with a magnet to remove it. The magnetism is a physical property. Also, the magnetite is heavier than the sand so the sand can be washed off as in gold pan, leaving mostly magnetite. 

The answer is a. the answer is a because they’re agreeing with other people’s op