Which best explains why blood and the cytoplasm of cells are mainly composed of liquid water?
a. water is a common substance and is readily available.
b. water is an important transport and reaction medium.
c. water prevents biological reactions from occurring readily.
d. water prevents turgor pressure from making the cells too rigid.

The correct answer is  B) Water is an important transport and reaction medium.

All the essential metabolic reactions for life take place in an aqueous medium. In other words, water acts as a solvent for the chemical reactions and also help in the transport of essential substances in and out of the cell.

Blood circulation takes place throughout the body and cytoplasm is present in all the cells.

Thus, to carry out the essential metabolic reactions and transport the substances, blood and cytoplasm is mainly composed of water.

So B) is the correct option.