Final answer:
Hyponatremia causes cells to swell as water enters them by osmosis, due to the lower sodium concentration in the ECF compared to the intracellular fluid.
Step-by-step explanation:
Hyponatremia refers to a condition where there is a relative decrease in blood sodium levels, often due to conditions like water retention or an imbalance in the body's sodium. At the cellular level, this leads to an increased entry of water into cells by osmosis, resulting in cell swelling. The correct answer to the question is A) swelling of cells, as the low sodium concentration in the extracellular fluid (ECF) compared to the intracellular fluid causes water to move into the cells, leading to their swelling.