If the solution surrounding a cell has a lower concentration of solutes than inside the cell, water will move into the cell through osmosis, causing it to expand. What kind of solution is surrounding the cell?

The solution surrounding the cell would be termed "hypotonic," in which the osmolarity, or concentration of small particles, is less that the osmolarity of the solution of reference (cytosol). This relative "lack of solids" outside as compared to inside, will lead to water diffusion to where there are more water molecules per solids, or less concentration of water. Osmosis is this driving force that occurs from the diffusion of water from a greater concentration of water (lesser solutes) to a lesser concentration of water (greater solutes). This phenomenon is crucial towards maintaining life and homeostasis within organisms.

Note: the term "osmolarity" refers to the concentration of solid dissolvable particles (solutes or osmolytes) as compared to the concentration of the solvent (water). Therefore it is similar to molarity, where the higher the osmolarity, then the more particles are present per volume of water; and likewise the lower the osmolarity, the more diluted a solution is. It's name comes from the very fact that differences in neighboring osmolarities contribute to osmosis, or the net movement of water.