The correct answer is (b).
When common salt is added to water, the boiling point of water increases and the freezing point of water decreases. This is because the salt ions interfere with the ability of the water molecules to form hydrogen bonds. Hydrogen bonds are what hold water molecules together in a liquid state. When the salt ions interfere with these bonds, it takes more energy to break them apart, which is why the boiling point of water increases. The freezing point of water decreases because the salt ions interfere with the ability of the water molecules to form ice crystals. When the salt ions interfere with these crystals, it takes less energy to form them, which is why the freezing point of water decreases.
Option (a) is incorrect because the boiling point of water increases when common salt is added. Option (c) is incorrect because the freezing point of water decreases when common salt is added. Option (d) is incorrect because the boiling point and freezing point of water both change when common salt is added, but they do not change in the same way.