To separate a solid substance that has dissolved in water, the most appropriate method would be to boil the water away, leaving the substance behind in the beaker. This method effectively separates the dissolved substance (assuming it is non-volatile) by removing the solvent (water) through evaporation.
The other options, such as using a filter or a magnet, would not be effective for a dissolved substance since it is no longer in solid form and cannot be trapped by a filter or attracted by a magnet.