The best way to separate a solid substance that has dissolved in water is to boil the water away, leaving the substance behind in the beaker. This method works because it takes advantage of the fact that the solid substance, being dissolved, will remain after the water has evaporated.
The other options either misunderstand the nature of dissolution (a filter won’t separate a dissolved substance, as it is in solution) or would not effectively separate the solid from the liquid (such as using a magnet for a non-magnetic dissolved substance).