Answer:
a) one substance must dissolve in another.
Step-by-step explanation:
A solution is mixture in which a substance (solid, liquid or gas) dissolves in a liquid. A salt dissolved in water is an example solid dissolved in liquid. Ammonia dissolved in water is an example of liquid in liquid and oxygen dissolved in water is an example gas dissolved in liquid.
The substance being dissolved is known as solute and the liquid in which solute is known as solvent.