Question

"The best explanation for why the onion cells do not burst, while red blood cells often do, is that

Answer 1

After seeing the attached picture represented above, it has become more clear and I think that the best explanation for why the onion cells do not burst, while red blood cells often do, is that the red blood cells have only a cell membrane, which does not protect them from bursting. The main point which makes difference between two parts of picture is that one of the cells has protective membrane.
Hope you will find it helpful.
Regards.

Answer 2

The onion cells are plant cells, and plant cells have cell membrane and cell wall both. The cell wall is composed of the cellulose, and it provides rigidity to the cell and helps the cell to maintain its shape. That is why when an onion cell is put in distilled water, it did not burst.

However, the red blood cells do not have cell wall. They only have cell membrane, which cannot provide rigidity to the cell, and therefore, it bursts when put in distilled water.

Hence, the correct answer is option (1) - 'the red blood cells only have cell membrane which does not protect them from bursting'.