The statement that best describes a solution is the option C: a mixture having a uniform composition where the components cannot be seen separately and all components are in the same state. That is exactly what a solution is: a homogeneous mixture, the composition is uniform, but it can vary from one solution to other. The components must be in the safe phase, but it can be any phase: solid, liquid or gas. The most classical and clear example is the salt solution, NaCl. When you dissolve a spoon of NaCl in water you will not be able to distinguish nor separating the solute from the solvent, and the mixture will have uniform composition.
