A) water, salt
B) salt, water
C) water, a variety of non-polar molecules
D) water, a variety of polar molecules
In order to answer the question, elimination of the choices would be necessary.
First, salt can never be a solvent in cells. So we can cancel option B. Water is the universal solvent and that makes it essential to humans since it helps in the overall cellular metabolism happening in the cells.
Water is a polar solvent and since we follow the term. "Like dissolves like", we know that it can only dissolve polar solutes. Salts (in relation to cellular biology) are polar molecules. With this statement we can cancel option C.
Now since water can dissolve polar molecules anything can be inside the cells. However that is not the case. Variations of polar molecules in the cells are only limited and most of it are salts. With that, we can choose letter A as the best answer. Oftentimes, cells are termed to have a "salty cytoplasm".
The answer is definietly not B, as water is the biological and universal solvent.