The correct answer is A. 18.00.
Molality is a measure of the concentration of a solution. It is defined as the number of moles of solute per kilogram of solvent. The molality of pure water is 18.00 m, which means that there are 18.00 moles of water per kilogram of water.
Option B, 1.80 m, is the molality of a solution that contains 1.80 moles of solute per kilogram of solvent. Option C, 5.55 m, is the molality of a solution that contains 5.55 moles of solute per kilogram of solvent. Option D, 55.5 m, is the molality of a solution that contains 55.5 moles of solute per kilogram of solvent.
The molality of a solution is important because it is a measure of the strength of the solution. The higher the molality, the stronger the solution. The molality of a solution is also important because it is used to calculate the freezing point depression and boiling point elevation of the solution.