Density is a physical property of every object that describes the mass per volume ratio. Using information about the mass (amount of stuff) and the volume (the amount of space) for two or more objects, one can make comparisions of their relatice densities.
 

In this question, you must rank the three aqueous solutions according to their relative molarity. In doing so, you must consider their ratio of moles of solute per liter of solution.  The volume of the solution is determined by the volume of water. Normally, both variables - moles and liters - are of equal consideration. But in this question, the volume of solution (L H₂O) is stated to be the same for all three solutions. Thus, the volume of solution becomes a non-variable. The decision as to which solution has the smallest and greatest molarity will be based on the number of moles of solute.

The identity of the solute varies from solution to solution and this is of great importance. The problem states the mass of solute (grams)  in each of the solutions. Mass is related to the number of moles of solute by the molar mass. Knowing the formula of the solute, you can determine the molar mass. This is most frequently determined using a Periodic Table but (luck you!) in this exercise we provide a table of molar mass values for all possible solutes. Tap on the View Molar Mass Values button and use the molar mass values (in grams per mole) to determine the number of moles of each solute. Since all three solutions use the same mass of solute, the solute with the smallest molar mass will have the greatest number of moles of solute. Since molarity is directly related to the number of moles, the solution with the greatest number of moles will also have the greatest molarity.