Molecule C would be least soluble in water because it is not a polar compound/molecule. It also lacks any unit that can combine with water to form a good hydrogen bond.
The general rule of solubility is that like dissolves likes.
Polar molecules will only dissolve in polar solvent.
Water is a polar solvent with hydrogen bonds in them.
Hydrocarbons typically have weak london dispersion forces between them and they are not polar.
Other compounds have functional groups that can form hydrogen bond with water.
