Final answer:
To separate rocks from a beaker of saltwater, filtration should be used to physically separate the rocks from the dissolved salt ions. While evaporation and other methods might be relevant for separating homogenous mixtures like saltwater, filtration is the most direct approach for this scenario.
Step-by-step explanation:
To separate rocks from a beaker of saltwater, the most effective method would be to filter the mixture to separate the rocks (Option B). This physical separation technique leverages the difference in particle size between the rocks and the dissolved salt ions. A filtration apparatus will allow the saltwater to pass through while the rocks remain on the filter paper. Conversely, if we were to evaporate the water (Option A), we would collect the salt, but this would not directly separate the rocks. Using a magnet (Option D) would only work if the rocks were magnetic, which is not typically the case. Lastly, dissolving the salt in the water is not a separation method, because salt is already dissolved in saltwater.
Chemistry informs us that the salt (sodium chloride) can't be separated from water by a physical process like filtration, but since rocks are not dissolved in the water, they can be separated this way. Homogeneous mixtures like saltwater can be separated into their components by taking advantage of the physical properties, such as boiling points; in the case of saltwater, through distillation or crystallization. However, for our case filtration is the appropriate method to remove the rocks from the saltwater.