The mixtures that could be separated by magnetism from your list are:
- nickel ore (if it contains magnetic nickel)
- iron-fortified vitamins (if they contain iron, which is magnetic)
The other mixtures, such as beach sand, trail mix, and freshly mined coal, do not generally contain ferromagnetic materials that would be attracted to a magnet. Thus, the applicable items for separation by magnetism are nickel ore and iron-fortified vitamins.