Final answer:
The correct option is b. hypotonic A 0.9% solution of salt in water is hypotonic relative to the interior of a cell, causing water to enter the cell and leading to cell swelling.
Step-by-step explanation:
A 0.9% solution of salt in water is hypotonic relative to the interior of a cell. In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell, meaning it has a lower concentration of solutes. This causes water to enter the cell, leading to cell swelling.
In contrast, an isotonic solution has the same osmolarity as the cell, resulting in no net movement of water into or out of the cell. A hypertonic solution, on the other hand, has a higher osmolarity than the cell, causing water to leave the cell and leading to cell shrinkage.